added some references to the doc

doc/filters/json_encode.rst

@@ -6,3 +6,9 @@ The ``json_encode`` filter returns the JSON representation of a string:
 .. code-block:: jinja
 
     {{ data|json_encode() }}
+
+.. note::
+
+    Internally, Twig uses the PHP `json_encode`_ function.
+
+.. _`json_encode`: http://php.net/json_encode

doc/filters/striptags.rst

@@ -10,7 +10,6 @@ by one space:
 
 .. note::
 
-    Internally, Twig uses the PHP `strip_tags`_ function to maintain index
-    association.
+    Internally, Twig uses the PHP `strip_tags`_ function.
 
 .. _`strip_tags`: http://php.net/strip_tags

doc/filters/url_encode.rst

@@ -6,3 +6,9 @@ The ``url_encode`` filter URL encodes a given string:
 .. code-block:: jinja
 
     {{ data|url_encode() }}
+
+.. note::
+
+    Internally, Twig uses the PHP `urlencode`_ function.
+
+.. _`urlencode`: http://php.net/urlencode