[1.6.x] Fixed #20962 -- Documented that template tag render() may raise exceptions.

Thanks Keryn Knight for the report.

Backport of e077224f4a from master

AUTHORS

@@ -268,6 +268,7 @@ answer newbie questions, and generally made Django that much better:
     Janos Guljas
     Thomas Güttler <hv@tbz-pariv.de>
     Horst Gutmann <zerok@zerokspot.com>
+    Bouke Haarsma <bouke@haarsma.eu>
     Antti Haapala <antti@industrialwebandmagic.com>
     Scot Hacker <shacker@birdhouse.org>
     dAniel hAhler

docs/howto/custom-template-tags.txt

@@ -477,8 +477,12 @@ Notes:
 
 * The ``render()`` method is where the work actually happens.
 
-* ``render()`` should never raise ``TemplateSyntaxError`` or any other
-  exception. It should fail silently, just as template filters should.
+* ``render()`` should generally fail silently, particularly in a production
+  environment where :setting:`DEBUG` and :setting:`TEMPLATE_DEBUG` are
+  ``False``. In some cases however, particularly if :setting:`TEMPLATE_DEBUG` is
+  ``True``, this method may raise an exception to make debugging easier. For
+  example, several core tags raise ``django.template.TemplateSyntaxError``
+  if they receive the wrong number or type of arguments.
 
 Ultimately, this decoupling of compilation and rendering results in an
 efficient template system, because a template can render multiple contexts