Update supported and required properties for the separator role

[melanierichards]

Following the proposal in #859

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[jnurthen]

link for review

[carmacleod]

Need to think about:

Authors SHOULD also provide the value of aria-valuemin if it is not 0 and the value of aria-valuemax if it is not 100. If missing or not a number, the implicit values of these attributes are as follows:
The implicit value of aria-valuemin is 0.
The implicit value of aria-valuemax is 100.
The implicit value of aria-valuenow is 50.

Delete the valuenow, and maybe reword the prose a bit.

[melanierichards]

Removed implicit value note for aria-valuenow. I think the rest of the verbiage seems fine but let me know what you're thinking @carmacleod!