Complete the versioninfo.md documentation file

Complete the description of the versioninfo mechanism, which
commit 37997ba adds in an
incomplete state.

src/build-helpers/versioninfo.md

@@ -116,19 +116,66 @@ The `versioninfo.sh` script then prints these items, one per line.
 The `build-helpers/versioninfo.m4` m4 include file
 --------------------------------------------------
 
-TODO: Write text.
+This defines the two parts of the m4 code:
+
+ * The m4 code which calls `versioninfo.sh`, parses its output, and
+ defines some m4 macros before `AC_INIT`:
+
+ * The `versioninfo_items` macro describes the sequence of
+  versioninfo items printed in lines by the `versioninfo.sh`
+  script
+
+ * The `versioninfo_init` macro runs the `versioninfo.sh` script
+  and parses its output, `m4_define`ing a `versioninfo_<ITEM>`
+  macro for each `ITEM` from `versioninfo_items`
+
+ * The `versioninfo_stampfile` macro contains the name of the
+  stamp file which caches the versioninfo items for use in
+  tarballs.
+
+ * The `versioninfo_init` macro is evaluated once, so that
+  `versioninfo.sh` is only run once as well.
+
+ The m4 macros with the parsed results can then be evaluated using
+ `m4_defn(...)` both in the `AC_INIT(...)` invocation for the
+ `PACKAGE_VERSION`, and also later for defining
+ `AVRDUDE_FULL_VERSION` and git commit information.
+
+ * The m4 code `AC_DEFUN`s the `VERSIONINFO_SETUP` macro, which in
+ turn defines `AC_SUBST` substitution and make variables for each
+ of the versioninfo items, and also defines a few
+ stubstitution-only (not make) variables for use in make rules in
+ `versioninfo.mk` and `GNUmakefile.in`, which write, read, parse,
+ and compare the versioninfo items as defined in
+ `versioninfo_items`.
 
 
 The `build-helpers/versioninfo.mk` Makefile include file
 --------------------------------------------------------
 
-TODO: Write text.
+To be `include`d into the top-level `Makefile.am`, this does two things:
+
+ * Hooks into the `dist` and `distcheck` targets to check the
+ recorded version information is the same one `versioninfo.sh`
+ would detect right now, and prevents builds with out of date
+ versioninfo items.
+
+ * Writes the stamp file with the versioninfo data when creating a
+ dist tarball.
 
 
 The `configure.ac` file
 -----------------------
 
-TODO: Write text.
+Before `AC_INIT`, uses m4 logic to evaluate what the `PACKAGE_VERSION`
+should be.
+
+Expands the `VERSIONINFO_SETUP` macro both to run its content and to
+force `aclocal` to pull in `versioninfo.m4` into `aclocal.m4` so that
+the m4 code from `versioninfo.m4` is evaluated before `AC_INIT`.
+
+Evaluates the `AVRDUDE_FULL_VERSION` message and adds it as both a C
+preprocessor macro and a autoconf substitution and make variable.
 
 
 The `GNUmakefile.in` and `GNUmakefile` files