Updates documentation based on a11y feedback

[mperrotti]

Updates documentation based on feedback from the Accessibility team:

• Adds more info about using accessible labeling in select dropdowns
• Fixes inaccessible Tooltip example

Closes https://github.com/github/primer/issues/998
Closes #2595

Merge checklist

• [n/a] Added/updated tests
• Added/updated documentation
• [n/a] Tested in Chrome
• [n/a] Tested in Firefox
• [n/a] Tested in Safari
• [n/a] Tested in Edge

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[github-actions]

size-limit report 📦

Path	Size
dist/browser.esm.js	82.8 KB (0%)
dist/browser.umd.js	83.45 KB (0%)

[joshblack]

Looks great! Love the changes for tooltip 🔥