[1.5.x] Clarified strip_tags documentation

The fact that strip_tags cannot guarantee to really strip all
non-safe HTML content was not clear enough. Also see:
https://www.djangoproject.com/weblog/2014/mar/22/strip-tags-advisory/

Partial backport (doc-only) of 6ca6c36f82 from master.

docs/ref/templates/builtins.txt

@@ -1944,7 +1944,7 @@ If ``value`` is ``10``, the output will be ``1.000000E+01``.
 striptags
 ^^^^^^^^^
 
-Strips all [X]HTML tags.
+Makes all possible efforts to strip all [X]HTML tags.
 
 For example::
 
@@ -1953,6 +1953,16 @@ For example::
 If ``value`` is ``"<b>Joel</b> <button>is</button> a <span>slug</span>"``, the
 output will be ``"Joel is a slug"``.
 
+.. admonition:: No safety guarantee
+
+    Note that ``striptags`` doesn't give any guarantee about its output being
+    entirely HTML safe, particularly with non valid HTML input. So **NEVER**
+    apply the ``safe`` filter to a ``striptags`` output.
+    If you are looking for something more robust, you can use the ``bleach``
+    Python library, notably its `clean`_ method.
+
+.. _clean: http://bleach.readthedocs.org/en/latest/clean.html
+
 .. templatefilter:: time
 
 time

docs/ref/utils.txt

@@ -616,15 +616,23 @@ escaping HTML.
 
 .. function:: strip_tags(value)
 
-    Removes anything that looks like an html tag from the string, that is
-    anything contained within ``<>``.
+    Tries to remove anything that looks like an HTML tag from the string, that
+    is anything contained within ``<>``.
+    Absolutely NO guaranty is provided about the resulting string being entirely
+    HTML safe. So NEVER mark safe the result of a ``strip_tag`` call without
+    escaping it first, for example with :func:`~django.utils.html.escape`.
 
     For example::
 
         strip_tags(value)
 
-    If ``value`` is ``"<b>Joel</b> <button>is</button> a <span>slug</span>"`` the
-    return value will be ``"Joel is a slug"``.
+    If ``value`` is ``"<b>Joel</b> <button>is</button> a <span>slug</span>"``
+    the return value will be ``"Joel is a slug"``.
+
+    If you are looking for a more robust solution, take a look at the `bleach`_
+    Python library.
+
+    .. _bleach: https://pypi.python.org/pypi/bleach
 
 .. function:: remove_tags(value, tags)