transaction: document startgroup and endgroup

These enigmatic methods are only used in repair. We document them to clarify
there purpose and user.

mercurial/transaction.py

@@ -117,10 +117,18 b' class transaction(object):'
 
     @active
     def startgroup(self):
+        """delay registration of file entry
+
+        This is used by strip to delay vision of strip offset. The transaction
+        sees either none or all of the strip actions to be done."""
         self._queue.append(([], []))
 
     @active
     def endgroup(self):
+        """apply delayed registration of file entry.
+
+        This is used by strip to delay vision of strip offset. The transaction
+        sees either none or all of the strip actions to be done."""
         q = self._queue.pop()
         self.entries.extend(q[0])
         self._backupentries.extend(q[1])