help: document about {extras} template keyword

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

mercurial/templatekw.py

@@ -219,6 +219,8 b' def showdiffstat(repo, ctx, templ, **arg'
     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()