update: correct description of --check option

The old "update across branches if no uncommitted changes" made
it sound like updating across branches (with no uncommitted changes)
was allowed only with this option, which was not true. Also, the option
did not care whether it was linear or across branches. Instead, it
checked that there were no uncommitted changes. Let's explain what it
does instead of trying to suggest what happens without it.

mercurial/commands.py

@@ -7101,8 +7101,7 b' def unbundle(ui, repo, fname1, *fnames, '
 
 @command('^update|up|checkout|co',
     [('C', 'clean', None, _('discard uncommitted changes (no backup)')),
-    ('c', 'check', None,
-     _('update across branches if no uncommitted changes')),
+    ('c', 'check', None, _('require clean working directory')),
     ('d', 'date', '', _('tipmost revision matching date'), _('DATE')),
     ('r', 'rev', '', _('revision'), _('REV'))
      ] + mergetoolopts,