serve: mark --stdio and --cmdserver as "(ADVANCED)" flags

This will hide them from the default --help output.

mercurial/commands.py

@@ -4586,8 +4586,8 b' def root(ui, repo):'
     ('', 'webdir-conf', '', _('name of the hgweb config file (DEPRECATED)'),
      _('FILE')),
     ('', 'pid-file', '', _('name of file to write process ID to'), _('FILE')),
-    ('', 'stdio', None, _('for remote clients')),
-    ('', 'cmdserver', '', _('for remote clients'), _('MODE')),
+    ('', 'stdio', None, _('for remote clients (ADVANCED)')),
+    ('', 'cmdserver', '', _('for remote clients (ADVANCED)'), _('MODE')),
     ('t', 'templates', '', _('web templates to use'), _('TEMPLATE')),
     ('', 'style', '', _('template style to use'), _('STYLE')),
     ('6', 'ipv6', None, _('use IPv6 in addition to IPv4')),