gnome.gtkdoc: Fix missing permitted kwargs

Found by https://bugzilla.gnome.org/show_bug.cgi?id=790998

mesonbuild/modules/gnome.py

@@ -723,7 +723,8 @@ This will become a hard error in the future.''')
     @permittedKwargs({'main_xml', 'main_sgml', 'src_dir', 'dependencies', 'install',
                       'install_dir', 'scan_args', 'scanobjs_args', 'gobject_typesfile',
                       'fixxref_args', 'html_args', 'html_assets', 'content_files',
-                      'mkdb_args', 'ignore_headers', 'include_directories'})
+                      'mkdb_args', 'ignore_headers', 'include_directories',
+                      'namespace', 'mode', 'expand_content_files'})
     def gtkdoc(self, state, args, kwargs):
         if len(args) != 1:
             raise MesonException('Gtkdoc must have one positional argument.')