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

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

--- a/mercurial/commands.py	Sun Dec 17 18:43:05 2017 +0900
+++ b/mercurial/commands.py	Sun Jan 07 15:56:58 2018 +0900
@@ -2690,7 +2690,8 @@
     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.