Documentation/clone: document ignored configuration variables

Documentation/clone: document ignored configuration variables

Due to limitations in the current implementation, some configuration
variables specified via 'git clone -c var=val' (or 'git -c var=val
clone') are ignored during the initial fetch and checkout.

Let the users know which configuration variables are known to be
ignored ('remote.origin.mirror' and 'remote.origin.tagOpt') under the
documentation of 'git clone -c', along with hints to use the options
'--mirror' and '--no-tags' instead.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>