builtin/show-ref: explicitly spell out different modes in synopsis

builtin/show-ref: explicitly spell out different modes in synopsis

The synopsis treats the `--verify` and the implicit mode the same. They
are slightly different though:

    - They accept different sets of flags.

    - The implicit mode accepts patterns while the `--verify` mode
      accepts references.

Split up the synopsis for these two modes such that we can disambiguate
those differences.

While at it, drop "--quiet" from the pattern mode's synopsis. It does
not make a lot of sense to list patterns, but squelch the listing output
itself. The description for "--quiet" is adapted accordingly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>