Update authoring requirement for aria-selected on options (#1719)

A different solution for #1661, discussed on 4/7 and summarized in this comment: #1661 (comment)

I wrangled the existing wording around single vs. multiselect a little because the new `SHOULD` text about `aria-selected` overlapped with the existing single vs. multi-select wording, so I consolidated a bit.

* update to author requirement for aria-selected on options
* minor cleanup
some odd white spacing fixes and inconsistent period/capitalization between list item sentences.
* and again with the spacing...
* fix spacing, update checked/selected value wording

Co-authored-by: Scott O'Hara <scottaohara@users.noreply.github.com>

index.html

@@ -6039,8 +6039,13 @@ <h5>Note regarding the ARIA 1.1 <code>none</code> role.</h5>
           Otherwise, if a user agent provides an implicit <sref>aria-selected</sref> value for an <rref>option</rref>, the value SHOULD be <code>false</code>.
         </p>
         <p>
-          Authors MAY indicate selection for <rref>option</rref> elements  using either <sref>aria-selected</sref> or <sref>aria-checked</sref>.
-          Some user interfaces indicate selection with <sref>aria-selected</sref> in single-select list boxes and with <sref>aria-checked</sref> in multi-select list boxes.
+          Authors SHOULD indicate selection for <rref>option</rref> elements using one of the following:
+        </p>
+        <ul>
+          <li>An <sref>aria-selected</sref> value of <code>true</code> on the selected option within a single-select <rref>listbox</rref>, and optionally <sref>aria-selected</sref> values of <code>false</code> on unselected options.</li>
+          <li>Either <sref>aria-selected</sref> or <sref>aria-checked</sref> on all options within a multi-select <rref>listbox</rref>, with a value of <code>true</code> on selected options, and a value of <code>false</code> on unselected options.</li>
+        </ul>
+        <p>
           Authors SHOULD NOT specify both <sref>aria-selected</sref> and <sref>aria-checked</sref> on <rref>option</rref> elements contained by the same <rref>listbox</rref> except in the extremely rare circumstances where all the following conditions are met:
         </p>
         <ul>