Make ease of use update to template documentation

Add the phrases 'multiline comment' and 'triple quotes' to docs
in the templates/#block-assignments section. This allows for new
users to find this alternative easily.

Author: ratchek

docs/templates.rst

@@ -1090,9 +1090,10 @@ Block Assignments
 
 Starting with Jinja 2.8, it's possible to also use block assignments to
 capture the contents of a block into a variable name.  This can be useful
-in some situations as an alternative for macros.  In that case, instead of
-using an equals sign and a value, you just write the variable name and then
-everything until ``{% endset %}`` is captured.
+in some situations as an alternative for macros. It can also be used to create
+multiline strings instead of triple quotes (''' and """), which Jinja does not
+support. In that case, instead of using an equals sign and a value, you just
+write the variable name and then everything until ``{% endset %}`` is captured.
 
 Example::