[util] Note CSS compatibility in --help-features

util/options.cc

@@ -304,7 +304,9 @@ shape_options_t::add_options (option_parser_t *parser)
   const gchar *features_help = "Comma-separated list of font features\n"
     "\n"
     "    Features can be enabled or disabled, either globally or limited to\n"
-    "    specific character ranges.\n"
+    "    specific character ranges.  The format for specifying feature settings\n"
+    "    follows.  All valid CSS font-feature-settings values other than 'normal'\n"
+    "    and 'inherited' are also accepted, though, not documented below.\n"
     "\n"
     "    The range indices refer to the positions between Unicode characters,\n"
     "    unless the --utf8-clusters is provided, in which case range indices\n"