Mark as experimental

doc/extdev/appapi.rst

@@ -597,6 +597,8 @@ handlers to the events.  Example:
 
    .. versionadded:: 1.6
 
+      As a **experimental** event
+
 .. event:: html-collect-pages (app)
 
    Emitted when the HTML builder is starting to write non-document pages.  You

sphinx/domains/__init__.py

@@ -242,7 +242,7 @@ class Domain(object):
 
     def check_consistency(self):
         # type: () -> None
-        """Do consistency checks."""
+        """Do consistency checks (**experimental**)."""
         pass
 
     def process_field_xref(self, pnode):