Un-hide --backend argument

doc/qutebrowser.1.asciidoc

@@ -65,6 +65,9 @@ show it.
 *--target* '{auto,tab,tab-bg,tab-silent,tab-bg-silent,window}'::
     How URLs should be opened if there is already a qutebrowser instance running.
 
+*--backend* '{webkit,webengine}'::
+    Which backend to use (webengine backend is EXPERIMENTAL!).
+
 === debug arguments
 *-l* '{critical,error,warning,info,debug,vdebug}', *--loglevel* '{critical,error,warning,info,debug,vdebug}'::
     Set loglevel

qutebrowser/qutebrowser.py

@@ -70,8 +70,8 @@ def get_argparser():
                         help="How URLs should be opened if there is already a "
                              "qutebrowser instance running.")
     parser.add_argument('--backend', choices=['webkit', 'webengine'],
-                        # help="Which backend to use.",
-                        help=argparse.SUPPRESS, default='webkit')
+                        help="Which backend to use (webengine backend is "
+                             "EXPERIMENTAL!).", default='webkit')
 
     parser.add_argument('--json-args', help=argparse.SUPPRESS)
     parser.add_argument('--temp-basedir-restarted', help=argparse.SUPPRESS)