Warn about IE8 + Respond.js + file:// problem in examples; fix #11019
JS docs: add cross-reference to Getting Started mobile modal section
Merge pull request #11156 from twbs/mobile-modals-xref

JS docs: add cross-reference to Getting Started mobile modal section
Merge pull request #11155 from twbs/ie8-file-proto-respond.js-examples-warnings

Warn about IE8 + Respond.js + file:// problem in examples