CodingGuidelines: document the API in *.h files

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>