document that non-:visible scrollspy targets are ignored
document that non-:visible scrollspy targets are ignored

BS3 commits: aca859a13fc2f538aed7fffdd4d28a5393a195ea

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