[PATCH 1 of 3 V2] help: document about {extras} template keyword

[Matthew Turk]

# HG changeset patch
# User Matthew Turk <matthewturk at gmail.com>
# Date 1384552434 18000
#      Fri Nov 15 16:53:54 2013 -0500
# Node ID 0c27d52646b082ce934ab744ad5f3a28d4f9119a
# Parent  aa80446aacc3b1574211649cd8f190250b6b04b3
help: document about {extras} template keyword

This adds text about the "extras" template keyword, similar to that proposed by
Yuya Nishihara previously.

diff -r aa80446aacc3 -r 0c27d52646b0 mercurial/templatekw.py
--- a/mercurial/templatekw.py	Wed Nov 06 12:53:39 2013 -0500
+++ b/mercurial/templatekw.py	Fri Nov 15 16:53:54 2013 -0500
@@ -219,6 +219,8 @@
     return '%s: +%s/-%s' % (len(stats), adds, removes)
 
 def showextras(**args):
+    """:extras: List of dicts with key, value entries of the 'extras'
+    field of this changeset."""
     templ = args['templ']
     for key, value in sorted(args['ctx'].extra().items()):
         args = args.copy()