help: document stringify templatefilter stable

help: document stringify templatefilter

--- a/mercurial/help/templates.txt	Tue Aug 17 17:40:53 2010 -0500
+++ b/mercurial/help/templates.txt	Thu Aug 19 10:59:37 2010 +0200
@@ -140,6 +140,9 @@
 
 :shortdate: Date. Returns a date like "2006-09-18".
 
+:stringify: Any type. Turns the value into text by converting values into
+    text and concatenating them.
+
 :strip: Any text. Strips all leading and trailing whitespace.
 
 :tabindent: Any text. Returns the text, with every line except the