identify: document -r. explicitly how to disable wdir scanning (issue5622)

mercurial/commands.py

@@ -2690,7 +2690,8 b' def identify(ui, repo, source=None, rev='
     a list of tags, and a list of bookmarks.
 
     When REV is not given, print a summary of the current state of the
-    repository.
+    repository including the working directory. Specify -r. to get information
+    of the working directory parent without scanning uncommitted changes.
 
     Specifying a path to a repository root or Mercurial bundle will
     cause lookup to operate on that repository/bundle.