mention that revlinks are broken on periodic/nightly builds

Refs #1013

master/docs/cfg-statustargets.texinfo

@@ -565,18 +565,21 @@ in change comments.
 
 @subheading revlink
 
-The @code{revlink} is similarly used to create links from revision
-ids to a web-view of your source control system. This will either be
-a string, a dict from string (repository ids) to strings, or a callable.
+The @code{revlink} is used to create links from revision IDs in the web status
+to a web-view of your source control system. The parametr's value must be a
+format string, a dict mapping a string (repository name) to format strings, or a callable.
 
-The string should use '%s' to insert
-the revision id in the url. I.e. for Buildbot on github: 
-@code{revlink='http://github.com/djmitche/buildbot/tree/%s'}
-(The revision id will be URL encoded before inserted in the replacement string)
+The format string should use '%s' to insert the revision id in the url.  For
+example, for Buildbot on github:
+@code{revlink='http://github.com/buildbot/buildbot/tree/%s'} (The revision ID
+will be URL encoded before inserted in the replacement string)
 
 The callable takes the revision id and repository argument, and should return
 an URL to the revision. 
 
+Note that SourceStamps that are not created from version-control changes (e.g.,
+those created by a Nightly or Periodic scheduler) will have an empty repository
+string, as the respository is not known.
 
 @subheading changecommentlink