Generate the command list in mmh-intro(7)

Generate the command list in mmh-intro(7)

This implementation does handle the aliases of folder(1) and
flist(1), but not the ones of show(1) and new(1). Hence the
commands next(1), prev(1), fnext(1), fprev(1) and unseen(1)
don't appear on the list.

Adjusted some of the short descriptions based on the version
in the manual command list.