Improved message when CL description is missing

'Add a description' is not enough to know what the test is about. Added
some more info.

BUG=

Review URL: https://codereview.chromium.org/313523002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@274393 0039d316-1c4b-4281-b951-d872f2087c98

presubmit_canned_checks.py

@@ -61,9 +61,9 @@ def CheckChangeHasDescription(input_api, output_api):
   text = input_api.change.DescriptionText()
   if text.strip() == '':
     if input_api.is_committing:
-      return [output_api.PresubmitError('Add a description.')]
+      return [output_api.PresubmitError('Add a description to the CL.')]
     else:
-      return [output_api.PresubmitNotifyResult('Add a description.')]
+      return [output_api.PresubmitNotifyResult('Add a description to the CL.')]
   return []