qapi: Check feature documentation against the schema

Commit f3ed93d545 "qapi: Allow documentation for features" neglected
to check documentation against the schema.  Fix that: check them the
same way we check arguments.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191024110237.30963-20-armbru@redhat.com>

scripts/qapi/schema.py

@@ -56,6 +56,8 @@ class QAPISchemaEntity(object):
         seen = {}
         for f in self.features:
             f.check_clash(self.info, seen)
+            if self.doc:
+                self.doc.connect_feature(f)
 
         self._checked = True