Document and test {#! syntax.

Fixes #1606

tornado/template.py

@@ -94,12 +94,15 @@ def add(x, y):
 Template expressions are surrounded by double curly braces: ``{{ ... }}``.
 The contents may be any python expression, which will be escaped according
 to the current autoescape setting and inserted into the output.  Other
-template directives use ``{% %}``.  These tags may be escaped as ``{{!``
-and ``{%!`` if you need to include a literal ``{{`` or ``{%`` in the output.
+template directives use ``{% %}``.
 
 To comment out a section so that it is omitted from the output, surround it
 with ``{# ... #}``.
 
+These tags may be escaped as ``{{!``, ``{%!``, and ``{#!``
+if you need to include a literal ``{{``, ``{%``, or ``{#`` in the output.
+
+
 ``{% apply *function* %}...{% end %}``
     Applies a function to the output of all template code between ``apply``
     and ``end``::

tornado/test/template_test.py

@@ -67,6 +67,7 @@ def test_escaping(self):
         self.assertRaises(ParseError, lambda: Template("{%"))
         self.assertEqual(Template("{{!").generate(), b"{{")
         self.assertEqual(Template("{%!").generate(), b"{%")
+        self.assertEqual(Template("{#!").generate(), b"{#")
         self.assertEqual(Template("{{ 'expr' }} {{!jquery expr}}").generate(),
                          b"expr {{jquery expr}}")