CodingGuidelines: document the API in *.h files

It makes it harder to let the API description and the reality drift apart if the doc is kept close to the implementation or the header of the API. We have been slowly migrating API docs out of the Documentation/technical/api-* to *.h files, and the development community generally considers that how inline docs in strbuf.h is done the best current practice. We recommend documenting in the header over documenting near the implementation to encourage people to write the docs that are readable without peeking at the implemention. Helped-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>