[1.5.x] Direct readers to format_html() in mark_safe() docs.

Backport of 4832c004e8 from master

docs/ref/utils.txt

@@ -762,6 +762,9 @@ appropriate entities.
 
     Can be called multiple times on a single string.
 
+    For building up fragments of HTML, you should normally be using
+    :func:`django.utils.html.format_html` instead.
+
     String marked safe will become unsafe again if modified. For example::
 
         >>> mystr = '<b>Hello World</b>   '