credential: move doc to credential.h

credential: move doc to credential.h

Move the documentation from Documentation/technical/api-credentials.txt
to credential.h as it's easier for the developers to find the usage
information beside the code instead of looking for it in another doc file.

Documentation/technical/api-credentials.txt is removed because the
information it has is now redundant and it'll be hard to keep it up to
date and synchronized with the documentation in the header file.

Documentation/git-credential.txt and Documentation/gitcredentials.txt now link
to credential.h instead of Documentation/technical/api-credentials.txt for
details about the credetials API.

Signed-off-by: Heba Waly <heba.waly@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>