Fixed #21316 -- Documented that modifying safe strings makes them unsafe.

    Can be called multiple times on a single string.

    String marked safe will become unsafe again if modified. For example::

        >>> mystr = '<b>Hello World</b>   '

        >>> mystr = mark_safe(mystr)

        >>> type(mystr)

        <class 'django.utils.safestring.SafeBytes'>

        >>> mystr = mystr.strip()  # removing whitespace

        >>> type(mystr)

        <type 'str'>

.. function:: mark_for_escaping(s)

    Explicitly mark a string as requiring HTML escaping upon output. Has no