docs: minor fixes to g.gisenv, i.atcorr, v.centroids, v.overlay, v.ty…

…pe (#4484)

- g.gisenv.html: add EXAMPLES section (MEMORYMB, NPROCS)
- i.atcorr.html: explain missing acronyms
- v.centroids.html: start intro with what a centroid means
- v.out.svg: fix GRASS_NS URL (fixes #4474)
- v.overlay figures: colorize selected polygon in yellow color, not grey (sync to v.select)
- v.type.html: explain vector object types in intro

Note that the v.overlay figures should be redone to improve their quality (in a different PR).

Co-authored-by: Anna Petrasova <kratochanna@gmail.com>

general/g.gisenv/g.gisenv.html

@@ -149,6 +149,32 @@ <h2>NOTES</h2>
 variables are stored in <tt>&lt;gisdbase&gt;/&lt;project&gt;/&lt;mapset&gt;/VAR</tt>
 after the current GRASS session is closed.
 
+<h2>EXAMPLES</h2>
+
+<h3>Cache for raster operations</h3>
+
+The maximum memory to be used, i.e. the cache size for raster rows, is set
+to 300 MB by default (GRASS variable <em>MEMORYMB</em>). To speed up
+raster operations, it is recommended to increase this setting if enough RAM
+is available. It is important to note that parallel processes will each
+consume this amount of RAM.
+
+Set the maximum memory to be used (in MB), i.e. the cache size for raster rows:
+
+<div class="code"><pre>
+# set to 6 GB (default: 300 MB)
+g.gisenv set="MEMORYMB=6000"
+</pre></div>
+
+<h3>Number of threads for parallel computing</h3>
+
+Set the number of threads for parallel computing:
+
+<div class="code"><pre>
+# set to use 12 threads (default: 1)
+g.gisenv set="NPROCS=12"
+</pre></div>
+
 <h3>GRASS Debugging</h3>
 
 To print debugging messages, the variable <em>DEBUG</em> must be set to level
@@ -189,7 +215,8 @@ <h2>SEE ALSO</h2>
 </em>
 
 <p>
-See also <a href="variables.html">variables list</a>
+See also <a href="variables.html#list-of-selected-grass-gisenv-variables">
+list of selected GRASS gisenv variables</a>
 
 <h2>AUTHOR</h2>
 

imagery/i.atcorr/i.atcorr.html

@@ -4,7 +4,7 @@ <h2>DESCRIPTION</h2>
 map using the 6S algorithm (<em>Second Simulation of Satellite Signal
 in the Solar Spectrum</em>). A detailed algorithm description is
 available at the
-<a href="http://modis-sr.ltdri.org/">Land Surface
+<a href="https://salsa.umd.edu/">Land Surface
 Reflectance Science Computing Facility website</a>.
 
 <p><em>Important: Current region settings are ignored!</em> The
@@ -507,6 +507,16 @@ <h3>F. Sensor band</h3>
 
 <p>Define your own spectral conditions:
 
+Note that "wlinf" and "wlsup" refer to the limits of the wavelength range
+defined by the user for a given simulation. Specifically:
+
+<ul>
+<li> wlinf: This represents the lower wavelength limit (or minimum wavelength)
+ of the spectral band for which the simulation is being performed.</li>
+<li> wlsup: This represents the upper wavelength limit (or maximum wavelength)
+ of the spectral band for the simulation.</li>
+</ul>
+
 <table border="1">
 
 <tr>

scripts/v.centroids/v.centroids.html

@@ -1,11 +1,13 @@
 <h2>DESCRIPTION</h2>
 
-GRASS defines vector areas as composite entities consisting of a set of
+In GRASS GIS, a centroid is a point within a closed ring of boundaries.
+A vector area is defined as composite entity consisting of a set of
 closed boundaries and a centroid. The attribute information associated
-with that area is linked to the centroid. The <em>v.centroids</em> module
-adds centroids to closed boundaries in the <b>input</b> file and assigns a
-category number to them. The starting value as well as the increment size
-may be set using optional parameters.
+with this area is linked to the centroid.
+
+The <em>v.centroids</em> module adds centroids to closed boundaries in
+the <b>input</b> file and assigns a category number to them. The starting
+value as well as the increment size may be set using optional parameters.
 <p>Multiple attributes may be linked to a single vector entity through
 numbered fields referred to as layers. Refer to <em>v.category</em>
 for more details, as <em>v.centroids</em> is simply a frontend to that

vector/v.out.svg/main.c

@@ -25,7 +25,7 @@
 
 #define SVG_NS "http://www.w3.org/2000/svg"
 #define XLINK_NS "http://www.w3.org/1999/xlink"
-#define GRASS_NS "http:/grass.itc.it/2006/gg"
+#define GRASS_NS "http://grass.itc.it/2006/gg"
 #define RADIUS_SCALE .003
 #define WIDTH_SCALE .001
 #define G_Areas "G_Areas"

vector/v.overlay/v.overlay.html

@@ -97,7 +97,7 @@ <h3>AND operator</h3>
 
 <center>
 <img src="v_overlay_op_and.png" alt="v.overlay with AND operator"><br>
-<i>Figure: v.overlay with AND operator (selected polygons in grey color)</i>
+<i>Figure: v.overlay with AND operator (selected polygons in yellow color)</i>
 </center>
 
 <h3>OR operator</h3>
@@ -113,7 +113,7 @@ <h3>OR operator</h3>
 
 <center>
 <img src="v_overlay_op_or.png" alt="v.overlay with OR operator"><br>
-<i>Figure: v.overlay with OR operator (selected polygons in grey color)</i>
+<i>Figure: v.overlay with OR operator (selected polygons in yellow color)</i>
 </center>
 
 <h3>XOR operator</h3>
@@ -129,7 +129,7 @@ <h3>XOR operator</h3>
 
 <center>
 <img src="v_overlay_op_xor.png" alt="v.overlay with XOR operator"><br>
-<i>Figure: v.overlay with XOR operator (selected polygons in grey color)</i>
+<i>Figure: v.overlay with XOR operator (selected polygons in yellow color)</i>
 </center>
 
 <h3>NOT operator</h3>
@@ -145,7 +145,7 @@ <h3>NOT operator</h3>
 
 <center>
 <img src="v_overlay_op_not.png" alt="v.overlay with NOT operator"><br>
-<i>Figure: v.overlay with NOT operator (selected polygon in grey color)</i>
+<i>Figure: v.overlay with NOT operator (selected polygon in yellow color)</i>
 </center>
 
 <h3>Overlay operations: AND, OR, NOT, XOR</h3>

vector/v.type/v.type.html

@@ -2,6 +2,27 @@ <h2>DESCRIPTION</h2>
 
 <em>v.type</em> changes the type of geometry primitives.
 
+<p>
+The following vector object types are defined in GRASS GIS:
+
+<ul>
+<li> point: a point;</li>
+<li> line: a directed sequence of connected vertices with two endpoints called nodes;</li>
+<li> boundary: the border line describing an area;</li>
+<li> centroid: a point within a closed ring of boundaries;</li>
+<li> area: the topological composition of a closed ring of boundaries and a centroid;</li>
+<li> face: a 3D area;</li>
+<li> kernel: a 3D centroid in a volume (not yet implemented);</li>
+<li> volume: a 3D corpus, the topological composition of faces and kernel (not yet implemented).</li>
+</ul>
+<p>
+Lines and boundaries can be composed of multiple vertices.
+<p>
+Area topology also holds information about isles. These isles are located
+within that area, not touching the boundaries of the outer area. Isles
+are holes inside the area, and can consist of one or more areas. They are
+used internally to maintain correct topology for areas.
+
 <h2>EXAMPLES</h2>
 
 <em>Convert lines to area boundaries</em><br>