Mention limitation of form validation feedback icons with input groups

BS3 commits: 13b4e7a1a8b9be8793c59e3c1da4016ad0974b56
Fixes 12610: Better and more consistent prefixing of docs CSS with .bs-docs-

BS3 commits: 0d49920e0dd08643238aabad11783d58ee72798d
Fixes 12625: Only remove first list group item's top and last list group item's bottom borders when the list group is first or last. grunt dist

BS3 commits: 44bb7b20c7eb14f29827fc163fc5afb066a7ddf5,
769339ed18afbaf24121e1b4e5a12d0b6f1d1828