add callout about justified button group borders to address 11964
add callout about justified button group borders to address 11964

BS3 commits: 71e9db41834ae1f9873e4047033f868be5e98036

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