Fixed #21881 -- Clarify behavior of {% ssi %} template tag.

When using the `parsed` option, {% ssi %} has the same limitation
as {% include %}, namely that the rendering of the included
template is done separately.

Thanks to trac user nagyv for the report and to alextreme for
the patch.

docs/ref/templates/builtins.txt

@@ -957,7 +957,7 @@ current page::
 The first parameter of ``ssi`` can be a quoted literal or any other context
 variable.
 
-If the optional "parsed" parameter is given, the contents of the included
+If the optional ``parsed`` parameter is given, the contents of the included
 file are evaluated as template code, within the current context::
 
     {% ssi '/home/html/ljworld.com/includes/right_generic.html' parsed %}
@@ -966,6 +966,13 @@ Note that if you use ``{% ssi %}``, you'll need to define
 :setting:`ALLOWED_INCLUDE_ROOTS` in your Django settings, as a security
 measure.
 
+.. note::
+    With the :ttag:`ssi` tag and the ``parsed`` parameter
+    there is no shared state between files -- each include is a completely
+    independent rendering process. This means it's not possible for example to
+    define blocks or alter the context in the current page using the included
+    file.
+
 See also: :ttag:`{% include %}<include>`.
 
 .. templatetag:: templatetag