Tweak language around modal dialogs

[patrickhlauke]

Closes #4185

Note that this only softens/expands some of the example language in the understanding documents. The original issue #4185 seems to conflate non-normative examples in a non-normative document with requirements of how modal dialogs should be implemented ... they are not. They provide examples of appropriate focus order and absence of keyboard traps. They are not the only ways in which focus order or absence of keyboard traps can be achieved. Examples are non-exhaustive, and there is no need - in my view - to expand these further to specifically address variations that result from different behaviours related to <dialog>

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[mbgower]

I'm a bit concerned about these changes. Pretty consistently in our examples, we are listing best behaviour/practices. As you note here (and in the issue), someone misconstruing an example with a normative edict shouldn't necessarily lead us to made the examples less clear or confusing (which I personally find these to be).
I'd prefer to leave as is, or lessen the change.