Documentation: document naming schema for structs and their functions

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>