Allowed/expected behavior of readonly on fieldset

[teoeter]

Related spec PR: whatwg/html#12424
Spec issue: whatwg/html#10293

The HTML spec is gaining a readonly content attribute on <fieldset>,
mirroring disabled. This raises a few ARIA-in-HTML questions:

1. Should aria-readonly on <fieldset> be permitted, prohibited, or
   considered redundant when the content attribute is present?
2. Should aria-readonly on a descendant <input>/<textarea> whose
   readonly state is inherited from a <fieldset readonly> ancestor
   be considered redundant? (Same question as for aria-disabled
   inside <fieldset disabled> today.)
3. Any role restrictions to call out?

Companion HTML AAM issue: w3c/html-aam#606