extdiff: document that it copies modified files back to working directory

Differential Revision: https://phab.mercurial-scm.org/D2976

hgext/extdiff.py

@@ -13,6 +13,11 b' diff programs are called with a configur'
 non-option arguments: paths to directories containing snapshots of
 files to compare.
 
+If there is more than one file being compared and the "child" revision
+is the working directory, any modifications made in the external diff
+program will be copied back to the working directory from the temporary
+directory.
+
 The extdiff extension also allows you to configure new diff commands, so
 you do not need to type :hg:`extdiff -p kdiff3` always. ::
 

tests/test-extension.t

@@ -730,6 +730,10 b' Extension module help vs command help:'
   called with a configurable set of options and two non-option arguments: paths
   to directories containing snapshots of files to compare.
   
+  If there is more than one file being compared and the "child" revision is the
+  working directory, any modifications made in the external diff program will be
+  copied back to the working directory from the temporary directory.
+  
   The extdiff extension also allows you to configure new diff commands, so you
   do not need to type 'hg extdiff -p kdiff3' always.