Document -Tgemini in output modes.  Also remove the SEE ALSO references, as they're more germane in their low-level function manpages.
Document -Tgemini in output modes.  Also remove the SEE ALSO references, as
they're more germane in their low-level function manpages.

References https://github.com/kristapsdz/lowdown/issues/57

kristapsdz/lowdown,kristapsdz/lowdown,kristapsdz/lowdown