Make tip help more helpful

mercurial/commands.py

@@ -2716,7 +2716,15 b' def tags(ui, repo):'
 def tip(ui, repo, **opts):
     """show the tip revision
 
-    Show the tip revision.
+    The tip revision (usually just called the tip) is the most
+    recently added changeset in the repository, the most recently
+    changed head.
+
+    If you have just made a commit, that commit will be the
+    tip. Alternately, if you have just pulled changes from another
+    repository, the tip of that repository becomes the current tip.
+    The "tip" tag is special and can not be renamed or assigned to a
+    different changeset.
     """
     cmdutil.show_changeset(ui, repo, opts).show(nullrev+repo.changelog.count())