[PATCH 5 of 6 V5] help: add local/other description to merge tool description

[Durham Goode]

# HG changeset patch
# User Durham Goode <durham at fb.com>
# Date 1400271307 25200
#      Fri May 16 13:15:07 2014 -0700
# Node ID 27cb0613b2f00a4638768454f272aa838f0503f0
# Parent  072a8cf959e8e51fb8009313046cda87829454c2
help: add local/other description to merge tool description

Adds a more detailed description of what 'local' and 'other' mean to the merge
tool documentation.

diff --git a/mercurial/help/config.txt b/mercurial/help/config.txt
--- a/mercurial/help/config.txt
+++ b/mercurial/help/config.txt
@@ -838,7 +838,13 @@
 ``args``
   The arguments to pass to the tool executable. You can refer to the
   files being merged as well as the output file through these
-  variables: ``$base``, ``$local``, ``$other``, ``$output``.
+  variables: ``$base``, ``$local``, ``$other``, ``$output``. The meaning
+  of ``$local`` and ``$other`` can vary depending on which action is being
+  performed. During and update or merge, ``$local`` represents the original
+  state of the file, while ``$other`` represents the commit you are updating
+  to or the commit you are merging with. During a rebase ``$local``
+  represents the destination of the rebase, and ``$other`` represents the
+  commit being rebased.
   Default: ``$local $base $other``
 
 ``premerge``