Added section and paragraph on accessibility considerations

[ianbjacobs]

in light of previous discussion of: https://github.com/w3c/payment-request/pull/450

closes #735

The following tasks have been completed:

• [x] Confirmed there are no ReSpec errors/warnings.

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Preview | Diff

[marcoscaceres]

Blocking on A11y Folk's approval.

[michael-n-cooper]

APA WG is accepting this proposal to address the comments solicited earlier, also issue #735.