Expand advice for optional icons with hidden labels

to clarify/hint that a ``<label class="sr-only">`` is not the only way
to do it
Split out/clarify ``<select>`` and ``multiple``
Callout for labelling of input groups

plus a few suggested approaches (in some cases, elided, as the actual
content of the label - being a generic example - is not important)
Use aria-label for pagination prev/next

Cleaner alternative to using ``<span class="sr-only">``
Make main segmented button keyboard-focusable

unless there's a reason why ``tabindex="-1"`` was dropped in there...
grunt-saucelabs now uses `throttled` instead of `concurrency`

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Merge pull request #15327 from patrickhlauke/docs-accessibility-tweaks2

More docs accessibility tweaks