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