Document example of using feedback icons with input groups.

Created by: le6o

It's not clear within the documentation that feedback icons should be placed outside of input groups when using add-ons.

If placed incorrectly on a horizontal form you get a positioning issue with the icon as seen here, http://jsfiddle.net/1s72tn38/1/

I was about to report this as a problem when I managed to find reference to placing the icon outside of the input-group within a related issue. https://github.com/twbs/bootstrap/issues/12551