Reword the "Conditions to expose sensor readings" section

[rakuco]

Follow-up to the review comments in #433.

The previous definition of "can expose sensor readings" was a bit too loose. It relied on a [=sensor type=] that was not passed to the operation and used "given document" to refer to the active document referenced in a prior paragraph.

Instead:

• Do away with all the "mandatory conditions" bits and only define "can expose sensor readings".
• Make said "can expose sensor readings" check explicitly require a Document. All parts of the spec that referenced this definition were already invoking it together with "the current browsing context's active document".
• Remove the checks related to the Permissions and Permissions Policy APIs. They required access to a sensor type that could not be easily retrieved by all callers; additionally, concrete Sensor classes all perform the Permissions Policy API checks in their constructors and Sensor.start() ensures the required permissions have been granted. We now make this explicit in a new note.

One possible improvement would be to normatively require all concrete Sensor classes to invoke "check sensor policy-controlled features" in their constructors, as this is currently done but not mandated.

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[rakuco]

@miketaylr would you mind taking a look at this one as a follow-up to your comment in #433?

[rakuco]

@anssiko could you merge this PR as well?

[anssiko]

Sure thing, thanks for pinging me.