Accessibility tweaks and explanation for glyphicons

- add aria-hidden="true" to avoid SRs unintentionally reading out the
Unicode character (see
http://www.filamentgroup.com/lab/bulletproof_icon_fonts.html)
- callout with advice on accessible icon usage
- replaced sr-only text in examples with (in the case of button) cleaner
aria-label
- additional example of icon used to convey meaning (in a
non-interactive control)
Meaningful icons in about page
Add aria-hidden to icons in about page
Merge pull request #15009 from patrickhlauke/glyphicon-accessibility-tweaks

Glyphicon accessibility tweaks