Documentation: document naming schema for structs and their functions

We nowadays have a proper mishmash of struct-related functions that are
called `<verb>_<struct>` (e.g. `clear_prio_queue()`) versus functions
that are called `<struct>_<verb>` (e.g. `strbuf_clear()`). While the
former style may be easier to tie into a spoken conversation, most of
our communication happens in text anyway. Furthermore, prefixing
functions with the name of the structure they operate on makes it way
easier to group them together, see which functions are related, and will
also help folks who are using code completion.

Let's thus settle on one style, namely the one where functions start
with the name of the structure they operate on.

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

Documentation/CodingGuidelines

@@ -541,6 +541,25 @@ For C programs:
    use your own debugger and arguments. Example: `GIT_DEBUGGER="ddd --gdb"
    ./bin-wrappers/git log` (See `wrap-for-bin.sh`.)
 
+ - The primary data structure that a subsystem 'S' deals with is called
+   `struct S`. Functions that operate on `struct S` are named
+   `S_<verb>()` and should generally receive a pointer to `struct S` as
+   first parameter. E.g.
+
+	struct strbuf;
+
+	void strbuf_add(struct strbuf *buf, ...);
+
+	void strbuf_reset(struct strbuf *buf);
+
+    is preferred over:
+
+	struct strbuf;
+
+	void add_string(struct strbuf *buf, ...);
+
+	void reset_strbuf(struct strbuf *buf);
+
 For Perl programs:
 
  - Most of the C guidelines above apply.