manifest: document return type of readfast()

I keep having to ponder out what readfast() means, and it always
surprises me. Document the return type in the docstring so that future
readers won't have to puzzle this out again.

mercurial/manifest.py

@@ -793,7 +793,13 b' class manifest(revlog.revlog):'
         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):