update docs about esc()

`null` is not `$context`

Author: totoprayogo1916

user_guide_src/source/general/common_functions.rst

@@ -81,7 +81,7 @@ Service Accessors
     If $data is a string, then it simply escapes and returns it.
     If $data is an array, then it loops over it, escaping each 'value' of the key/value pairs.
 
-    Valid context values: html, js, css, url, attr, raw, null
+    Valid context values: html, js, css, url, attr, raw
 
 .. php:function:: helper($filename)