doc: move credential helper info into gitcredentials(7)

doc: move credential helper info into gitcredentials(7)

The details of how credential helpers can be called or implemented were
originally covered in Documentation/technical/. Those are topics that
end users might care about (and we even referenced them in the
credentials manpage), but those docs typically don't ship as part of the
end user documentation, making them less useful.

This situation got slightly worse recently in f3b9055624 (credential:
move doc to credential.h, 2019-11-17), where we moved them into the C
header file, making them even harder to find.

So let's move put this information into the gitcredentials(7)
documentation, which is meant to describe the overall concepts of our
credential handling. This was already pointing to the API docs for these
concepts, so we can just include it inline instead.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>