Perhaps I'm a little slow & need to have things spelled out :)
https://github.com/twbs/bootstrap/issues/17301#issuecomment-134469886

Fix #17052

See https://github.com/twbs/bootstrap-sass/pull/805

Fixes #13583: Move .sr-only and .sr-only-focusable to mixins, then use them to create classes in _utilities.scss

This reverts commit d29f851e.

Making <img>s responsive by-default without opt-in can severely break
third-party widgets such as Google Maps.
This was an acknowledged problem in Bootstrap v2
(see https://github.com/twbs/bootstrap/issues/1506 ) and was fixed in v3
by requiring the .img-responsive class for explicit opt-in
(see 09cdee2f).
The situation hasn't really changed since then. The responsive-by-default
approach hasn't become any more suitable in the intervening time.

So let's avoid having this regress in v4. :-)

Based on http://css-tricks.com/inheriting-box-sizing-probably-slightly-better-best-practice/
This allows us to significantly simplify the relevant part of the docs.