explain 'none' more fully

[equalsJeffH]

If I understand correctly, a policy directive of, say:

foo-feature-name 'none'

..means that "foo feature" is disabled for the relevant browsing context and all of its nested browsing contexts (if any).

Whether or not the above is fully correct, the permissions policy spec currently does not explicitly explain 'none' AFAICT.

Also, IIUC, it is within the parse policy directive alg where 'none' takes effect. If that's correct, then it would be helpful to add a note in that alg (e.g., near step 2.9.3.1) saying something like:

If element is an ASCII case-insensitive match for "'none'" then allowlist remains empty, resulting in directive[feature] having an empty allowlist as its value, effectively disabling the feature for the relevant browsing context.