--- a/mercurial/manifest.py	Sat Oct 11 01:17:40 2014 -0700
+++ b/mercurial/manifest.py	Tue Apr 28 12:31:30 2015 -0400
@@ -793,7 +793,13 @@
         return self._newmanifest(d)
 
     def readfast(self, node):
-        '''use the faster of readdelta or read'''
+        '''use the faster of readdelta or read
+
+        This will return a manifest which is either only the files
+        added/modified relative to p1, or all files in the
+        manifest. Which one is returned depends on the codepath used
+        to retrieve the data.
+        '''
         r = self.rev(node)
         deltaparent = self.deltaparent(r)
         if deltaparent != revlog.nullrev and deltaparent in self.parentrevs(r):