docs: Differentiate between "None" and `NONE` in directives

[janbrasna]

Reading through the docs:

DIRECTIVES
A dictionary of policy directives. Each key in the dictionary is a directive and the value is a list of sources for that directive. The following is a list of all the directives that can be configured.

• default-src
  Set the default-src directive. A tuple or list of values, e.g.: ("'self'", 'cdn.example.net'). [“‘self’”]
• script-src
  Set the script-src directive. A tuple or list. None

For a while I wasn't sure of the output. If I omit script-src, the default None is meant as empty, no output, right? Not NONE as in [“‘none’”], which means something different?

Any idea how to disambiguate specifically for this case?