Clarify requirement for AP in annotation dictionaries

[MPBailey]

The Value cell for AP in table 166 explicitly says "(Optional; PDF 1.2)", but then goes on to say that "Every annotation [...], except for the two cases listed below, shall have at least one appearance dictionary."

I recommend changing the initial parenthetical text to something that make people realise that AP is not simply optional and that they need to read the full text; perhaps "(Usually required; PDF 1.2)".

[petervwyatt]

PDF TWG request wording change to: "(Required except for conditions listed below...)"

(PDF TWG would like more specific information instead of just "usually"...)