remote: move doc to remote.h and refspec.h

remote: move doc to remote.h and refspec.h

Move the documentation from Documentation/technical/api-remote.txt to
remote.h and refspec.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.

N.B. The doc for both push and fetch members of the remote struct aren't
moved because they are out of date, as the members were changed from arrays
of rspecs to struct refspec 2 years ago.

Also documentation/technical/api-remote.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.

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