[PATCH 3 of 5 relative-diff] patch.trydiff: add a docstring

[Siddharth Agarwal]

# HG changeset patch
# User Siddharth Agarwal <sid0 at fb.com>
# Date 1426622775 25200
#      Tue Mar 17 13:06:15 2015 -0700
# Node ID 194f4ddf99b761fbb2bc883cc93629ecc09c0c84
# Parent  11254d47861cfbda255bd76c86c22cc959c79041
patch.trydiff: add a docstring

It took me a bit to figure out what this function actually does.

diff --git a/mercurial/patch.py b/mercurial/patch.py
--- a/mercurial/patch.py
+++ b/mercurial/patch.py
@@ -2206,6 +2206,12 @@
 
 def trydiff(repo, revs, ctx1, ctx2, modified, added, removed,
             copy, getfilectx, opts, losedatafn, prefix):
+    '''given input data, generate a diff and yield it in blocks
+
+    If generating a diff would lose data like flags or binary data and
+    losedatafn is not None, it will be called.
+
+    prefix is added to every path in the diff output.'''
 
     def gitindex(text):
         if not text: