Fixed #17719 -- Documented that template syntax sequences cannot be used as string literals.

Every context contains ``True``, ``False`` and ``None``. As you would expect,

these variables resolve to the corresponding Python objects.

Limitations with string literals

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Django's template language has no way to escape the characters used for its own

syntax. For example, the :ttag:`templatetag` tag is required if you need to

output character sequences like ``{%`` and ``%}``.

A similar issue exists if you want to include these sequences in template filter

or tag arguments. For example, when parsing a block tag, Django's template

parser looks for the first occurrence of ``%}`` after a ``{%``. This prevents

the use of ``"%}"`` as a string literal. For example, a ``TemplateSyntaxError``

will be raised for the following expressions::

  {% include "template.html" tvar="Some string literal with %} in it." %}

  {% with tvar="Some string literal with %} in it." %}{% endwith %}

The same issue can be triggered by using a reserved sequence in filter

arguments::

  {{ some.variable|default:"}}" }}

If you need to use strings with these sequences, store them in template

variables or use a custom template tag or filter to workaround the limitation.

Playing with Context objects

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