update: fix help regarding update to ancestor

As pointed out on the user mailing list, hg will update just as happily
to an ancestor if there are uncommitted changes as to a descendant.

mercurial/commands.py

@@ -5607,10 +5607,10 b' def update(ui, repo, node=None, rev=None'
     current named branch and move the current bookmark (see :hg:`help
     bookmarks`).
 
-    If the changeset is not a descendant of the working directory's
-    parent, the update is aborted. With the -c/--check option, the
-    working directory is checked for uncommitted changes; if none are
-    found, the working directory is updated to the specified
+    If the changeset is not a descendant or ancestor of the working
+    directory's parent, the update is aborted. With the -c/--check
+    option, the working directory is checked for uncommitted changes; if
+    none are found, the working directory is updated to the specified
     changeset.
 
     Update sets the working directory's parent revison to the specified