verify: document the `err` method

Simple method get simple documentation.

mercurial/verify.py

@@ -57,6 +57,7 b' class verifier(object):'
         self.warnings += 1
 
     def err(self, linkrev, msg, filename=None):
+        """record a "error" level issue"""
         if linkrev is not None:
             self.badrevs.add(linkrev)
             linkrev = "%d" % linkrev