narrowrepo: add docstring for narrowpats

Differential Revision: https://phab.mercurial-scm.org/D2184

hgext/narrow/narrowrepo.py

@@ -75,6 +75,10 b' def wraprepo(repo, opts_narrow):'
 
         @localrepo.repofilecache(narrowspec.FILENAME)
         def narrowpats(self):
+            """matcher patterns for this repository's narrowspec
+
+            A tuple of (includes, excludes).
+            """
             return narrowspec.load(self)
 
         @localrepo.repofilecache(narrowspec.FILENAME)