Document `.icon-prev` and `.icon-next` for Carousel

Created by: zacechola

.icon-prev and .icon-next were added way back when to allow carousel to not depend on Glyphicons.

https://github.com/twbs/bootstrap/blob/3487898a6af8c1d5b12f7b040d30e71b19e34923/less/carousel.less#L127-L166

See: #8402 and #8517

They should be documented as alternatives.