explicitly doc that icon classes should only be used on empty elements
explicitly doc that icon classes should only be used on empty elements

BS3 commits: 47a3c6d60b5348ac68f5a0b6d6e6e7f490844cee

todc/todc-bootstrap,todc/todc-bootstrap,mozillo/todc-bootstrap,up2vs/todc-bootstrap,2947721120/todc-bootstrap,ztx1491/todc-bootstrap,aaroncnhk/todc-bootstrap,todc/todc-bootstrap,2947721120/googlebt4