verify: document the `warn` method

This is the first bit of an effort to document and augment the verify code.

mercurial/verify.py

@@ -52,6 +52,7 b' class verifier(object):'
         self.warnorphanstorefiles = True
 
     def warn(self, msg):
+        """record a "warning" level issue"""
         self.ui.warn(msg + "\n")
         self.warnings += 1