Fix #229: Document constant NONE vs Python's None

[robhudson]

No description provided.

[robhudson]

/cc @janbrasna

[janbrasna]

Lovely. I actually after several attempts to improve the docs realized that the info was there all the time, just in a different place:

django-csp/docs/configuration.rst

Lines 75 to 84 in ed8b965

	In the above example, the constant ``NONE`` is converted to the CSP keyword ``"'none'"`` and
	is distinct from Python's ``None`` value. The CSP keyword ``'none'`` is a special value that
	signifies that you do not want any sources for this directive. The ``None`` value is a
	Python keyword that represents the absence of a value and when used as the value of a directive,
	it will remove the directive from the policy.
	This is useful when using the ``@csp_replace`` decorator to effectively clear a directive from
	the base configuration as defined in the settings. For example, if the Django settings the
	``frame-ancestors`` directive is set to a list of sources and you want to remove the
	``frame-ancestors`` directive from the policy for this view:

But this addition is a great improvement nonetheless, for those not reading the doc linearly and just skimming through the examples/defaults etc., to help avoiding this quite an unfortunate confusion;)

[janbrasna]

(only adding a few naïve questions out of ignorance…)