Add comments in carousel.js to clarify that "slid" is not a typo

Addresses #13248.
Merge pull request #13249 from twbs/slid-is-not-a-typo

Add comments in carousel.js to clarify that "slid" is not a typo
explicitly doc that icon classes should only be used on empty elements; closes #13240
Merge pull request #13241 from twbs/address-13240

explicitly doc that icon classes should only be used on empty elements
Update "grunt-contrib-concat" dependency.
Run `grunt`.
re-deprecate @screen-xs-min per #12913

By definition, XS screens have no minimum size, only a maximum size.
(Symmetrically, LG screens have no maximum size, hence why there's no @screen-lg-max.)
help avoid misuse of Carousel's .active when copy-pasting from examples; fix #13252