libbpf: Add API documentation convention guidelines

This adds a section to the documentation for libbpf
naming convention which describes how to document
API features in libbpf, specifically the format of
which API doc comments need to conform to.

Signed-off-by: Grant Seltzer <grantseltzer@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20211004215644.497327-1-grantseltzer@gmail.com