All - Relax "must understand" language in manner parallel to JOSE changes

[Michael Jones]

There are lots of places where we say things like "Other members MAY be present if they are understood by both parties."

I propose that relax this to just say "Other members MAY be present", bringing our "must understand" treatment in line with that going into the JOSE specs.

Alternatively, we could be more specific and say "Other members MAY be present and SHOULD be ignored, if not understood".

[Michael Jones]

We decided to go with the MUST be ignored construction.

[Michael Jones]

Fixed #816 - Changed "must understand" language to "MUST be ignored if not understood".

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