unshelve: correct help string; unshelve does not accept file list
The non-option arguments to `hg unshelve` are interpreted as the name of the
shelve to unshelve, not the list of files to unshelve. While that functionality
would probably be nice to have, that's well beyond the scope of this
documentation fix.
Differential Revision: https://phab.mercurial-scm.org/D7365