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>

Author: HebaWaly

Documentation/git-credential.txt

@@ -19,8 +19,7 @@ from system-specific helpers, as well as prompting the user for
 usernames and passwords. The git-credential command exposes this
 interface to scripts which may want to retrieve, store, or prompt for
 credentials in the same manner as Git. The design of this scriptable
-interface models the internal C API; see
-link:technical/api-credentials.html[the Git credential API] for more
+interface models the internal C API; see credential.h for more
 background on the concepts.
 
 git-credential takes an "action" option on the command-line (one of

Documentation/gitcredentials.txt

@@ -186,8 +186,7 @@ CUSTOM HELPERS
 --------------
 
 You can write your own custom helpers to interface with any system in
-which you keep credentials. See the documentation for Git's
-link:technical/api-credentials.html[credentials API] for details.
+which you keep credentials. See credential.h for details.
 
 GIT
 ---