transaction: document the contents of `tr.backupentries`

transaction: document the contents of `tr.backupentries`

Now that all items are known we can document it.

--- a/mercurial/transaction.py	Wed Nov 05 01:33:16 2014 +0000
+++ b/mercurial/transaction.py	Wed Nov 05 01:30:29 2014 +0000
@@ -83,6 +83,7 @@
         self.onclose = onclose
         self.onabort = onabort
         self.entries = []
+        # a list of ('path', 'backuppath') entries.
         self.backupentries = []
         self.map = {}
         self.backupmap = {}