Documentation: document idiomatic function names

We semi-regularly have discussions around whether a function shall be
named `S_release()`, `S_clear()` or `S_free()`. Indeed, it may not be
obvious which of these is preferable as we never really defined what
each of these variants means exactly.

Carve out a space where we can add idiomatic names for common functions
in our coding guidelines and define each of those functions. Like this,
we can get to a shared understanding of their respective semantics and
can easily point towards our style guide in future discussions such that
our codebase becomes more consistent over time.

Note that the intent is not to rename all functions which violate these
semantics right away. Rather, the intent is to slowly converge towards a
common style over time.

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

Documentation/CodingGuidelines

@@ -560,6 +560,23 @@ For C programs:
 
 	void reset_strbuf(struct strbuf *buf);
 
+ - There are several common idiomatic names for functions performing
+   specific tasks on a structure `S`:
+
+    - `S_init()` initializes a structure without allocating the
+      structure itself.
+
+    - `S_release()` releases a structure's contents without freeing the
+      structure.
+
+    - `S_clear()` is equivalent to `S_release()` followed by `S_init()`
+      such that the structure is directly usable after clearing it. When
+      `S_clear()` is provided, `S_init()` shall not allocate resources
+      that need to be released again.
+
+    - `S_free()` releases a structure's contents and frees the
+      structure.
+
 For Perl programs:
 
  - Most of the C guidelines above apply.