Role spinbutton: allow empty values, no min, no max, and structure with sibling steppers

[mcking65]

For issue #797, allow empty spin buttons by:

1. Changing aria-valuenow to supported from required property.
2. Changing prose to author SHOULD from author MUST specify value for aria-valuenow.
3. Adding aria-valuetext as supported property.

For issue #642, allow min and max to be undefined:

1. Change aria-valuemin and aria-valuemax from required to supported properties.
2. Revise prose of paragraph with normative statements.

For issue #812:

1. Change description to clearly state that a text field with sibling buttons outside the spinbutton is permitted.
2. Related editorial change to consolidate keyboard requirements into a single paragraph.
3. Other related editorial revisions for clarity.

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[joanmarie]

For future reference, let's aim for one pull request per issue.

From my initial triage of what we can already commit to the stable (Working Draft) branch, the issue #812 stuff doesn't strike me as something for which we need Core-AAM changes or tests; it's purely authoring.

On the other hand, this same pull request changed the default value of aria-valuenow from 0 to "that there is no current value" which should be tested. There may also be the need for a corresponding change in the Core AAM. Adding aria-valuetext as a supported property shouldn't need a change in the Core-AAM, but will need testing.

Do we block merging this entire set of changes into stable because of two single-line (but significant) changes? Do we manually break apart the changes and commit parts, but leave parts out? Neither is good imho. Solution: Please don't do this in the future. :) :)