Add tag documentation to man page.

Please update the man page to include the tag descriptions in #168 (partially fixes #77 ). There's no reason to expect that someone trying to use the CLI will necessarily have access to https://github.com/linuxmint/timeshift/blob/master/README.md so when looking at the man page, there's no elucidation of what these letters mean:

TIMESHIFT(1)                                        User Commands                                       TIMESHIFT(1)

NAME
     Timeshift - manual page for Timeshift 25.12.4

DESCRIPTION
     Timeshift v25.12.4 by Tony George (teejeetech@gmail.com)
⋮
     --tags {O,B,H,D,W,M}
            Add tags to snapshot (default: O)

Thanks!

Start with the Timeshift man page and review the tag descriptions referenced in issue #168. Add explanations for the O, B, H, D, W, and M tag letters alongside the --tags option; done means CLI users can understand these values without consulting the README.