docs: document credential.helper allowed values

docs: document credential.helper allowed values

gitcredentials(7) already mentions several possible invocations that one
can use as the value for credential.helper.  However, many people are
not aware that there are other options than a simple credential helper
name, so let's place some explanatory text in the documentation for
credential.helper as well.

We still refer the user to gitcredential(7) for additional explanations
and helpful examples.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>