doc txt & -h consistency: add missing options and labels

Fix various issues of SYNOPSIS and -h output syntax where:

 * Options such as --force were missing entirely
 * ...or the short option, such as -f

 * We said "opts" or "options", but could instead enumerate
   the (small) set of supported options

 * Options that were missing entirely (ls-remote's --sort=<key>)

   As we can specify "--sort" multiple times (it's backed by a
   string-list" it should really be "[(--sort=<key>)...]", which is
   what "git for-each-ref" lists it as, but let's leave that issue for
   a subsequent cleanup, and stop at making these consistent. Other
   "ref-filter.h" users share the same issue, e.g. "git-branch.txt".

 * For "verify-tag" and "verify-commit" we were missing the "--raw"
   option.

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

builtin/diff-index.c

@@ -9,7 +9,7 @@
 #include "submodule.h"
 
 static const char diff_cache_usage[] =
-"git diff-index [-m] [--cached] "
+"git diff-index [-m] [--cached] [--merge-base] "
 "[<common-diff-options>] <tree-ish> [<path>...]"
 "\n"
 COMMON_DIFF_OPTIONS_HELP;