Documentation: document difference between release and free

We semi-regularly have discussions around whether a function shall be
named `release()` or `free()`. For most of the part we use these two
terminologies quite consistently though:

  - `release()` only frees internal state of a structure, whereas the
    structure itself is not free'd.

  - `free()` frees both internal state and the structure itself.

Carve out a space where we can add idiomatic names for common functions
in our coding guidelines. This space can get extended in the future when
we feel the need to document more idiomatic names.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -560,6 +560,18 @@ For C programs:
 
 	void reset_strbuf(struct strbuf *buf);
 
+ - There are several common idiomatic names for functions performing
+   specific tasks on structures:
+
+    - `<struct>_init()` initializes a structure without allocating the
+      structure itself.
+
+    - `<struct>_release()` releases a structure's contents without
+      freeing the structure.
+
+    - `<struct>_free()` releases a structure's contents and frees the
+      structure.
+
 For Perl programs:
 
  - Most of the C guidelines above apply.