quota: update Kconfig comment

f2fs support quota since commit 0abd675e ("f2fs: support plain
user/group quota"), let's document it.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230413151412.30059-1-frank.li@vivo.com>