doc: unify help text for "--message" option

This patch changes help text for "--message" option of commands below
for unification.

  - sign (of gpg)
  - tag

This unification reduces translation cost, too.

This patch doesn't change the description for "--message" of "hg
rebase" below, because this should contain "collapse" word to explain
its purpose (only for "--collapse") clearly.

    use text as collapse commit message

hgext/gpg.py

@@ -203,7 +203,7 b' def keystr(ui, key):'
           ('k', 'key', '',
            _('the key id to sign with'), _('ID')),
           ('m', 'message', '',
-           _('commit message'), _('TEXT')),
+           _('use text as commit message'), _('TEXT')),
           ('e', 'edit', False, _('invoke editor on commit messages')),
          ] + commands.commitopts2,
          _('hg sign [OPTION]... [REV]...'))

mercurial/commands.py

@@ -5696,7 +5696,7 b' def summary(ui, repo, **opts):'
     ('', 'remove', None, _('remove a tag')),
     # -l/--local is already there, commitopts cannot be used
     ('e', 'edit', None, _('edit commit message')),
-    ('m', 'message', '', _('use <text> as commit message'), _('TEXT')),
+    ('m', 'message', '', _('use text as commit message'), _('TEXT')),
     ] + commitopts2,
     _('[-f] [-l] [-m TEXT] [-d DATE] [-u USER] [-r REV] NAME...'))
 def tag(ui, repo, name1, *names, **opts):