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>