help: correct usage & behavior of "git help --guides"

help: correct usage & behavior of "git help --guides"

As noted in 65f98358c0c (builtin/help.c: add --guide option,
2013-04-02) and a133737b809 (doc: include --guide option description
for "git help", 2013-04-02) which introduced the --guide option, it
cannot be combined with e.g. <command>.

Change the command and the "SYNOPSIS" section to reflect that desired
behavior. Now that we assert this in code we don't need to
exhaustively describe the previous confusing behavior in the
documentation either, instead of silently ignoring the provided
argument we'll now error out.

The "We're done. Ignore any remaining args" comment added in
15f7d494380 (builtin/help.c: split "-a" processing into two,
2013-04-02) can now be removed, it's obvious that we're asserting the
behavior with the check of "argc".

The "--config" option is still missing from the synopsis, it will be
added in a subsequent commit where we'll fix bugs in its
implementation.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>