glossary: add deprecated and experimental

mercurial/help/glossary.txt

@@ -177,6 +177,9 b' DAG'
     Mercurial, the DAG is limited by the requirement for children to
     have at most two parents.
 
+Deprecated
+    Feature removed from documentation, but not scheduled for removal.
+
 Default branch
     See 'Branch, default'.
 
@@ -217,6 +220,9 b' Draft'
     repositories and may thus be safely changed by history-modifying
     extensions. See :hg:`help phases`.
 
+Experimental
+    Feature that may change or be removed at a later date.
+
 Graph
     See DAG and :hg:`log --graph`.