Updated instances of contradiction regarding focus management

[MelSumner]

Summary

If merged, this PR resolves issue #1987

Description

Changed instances where spec contradicted itself with respect to focus management as a responsibility of authors.

Per managing focus instructions:

Authors MUST manage focus on the following container roles:

grid listbox menu menubar radiogroup tree treegrid tablist

I've updated these places where SHOULD was used in the keyboard accessibility sections instead of MUST.

NOTE: there are still two places where SHOULD was left in, because they are not part of this list: dialog and spinbutton.

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