[PATCH 6 of 7 🚂] transaction: document `tr.add`

[Pierre-Yves David]

# HG changeset patch
# User Pierre-Yves David <pierre-yves.david at fb.com>
# Date 1415192448 0
#      Wed Nov 05 13:00:48 2014 +0000
# Node ID 7a4e0a8524b5efbf8eb0c6848bf316bfdfaa325b
# Parent  d648190dde0ef6d2378700bfc780147ff782fe1e
transaction: document `tr.add`

diff --git a/mercurial/transaction.py b/mercurial/transaction.py
--- a/mercurial/transaction.py
+++ b/mercurial/transaction.py
@@ -140,10 +140,11 @@ class transaction(object):
         self.file.write(d)
         self.file.flush()
 
     @active
     def add(self, file, offset, data=None):
+        """record the state of an append-only file before update"""
         if file in self.map or file in self._backupmap:
             return
         if self._queue:
             self._queue[-1].append((file, offset, data))
             return