docs: indicate new credential protocol fields

Now that we have new fields (authtype and credential), let's document
them for users and credential helper implementers.

Indicate specifically what common values of authtype are and what values
are allowed.  Note that, while common, digest and NTLM authentication
are insecure because they require unsalted, uniterated password hashes
to be stored.

Tell users that they can continue to use a username and password even if
the new capability is supported.

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

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