improve affix docs with step-by-step walk through of plugin behavior and required CSS
improve affix docs with step-by-step walk through of plugin behavior and required CSS

BS3 commits: 1e6c95cd5d04ae5750d46195d652e21e9a461647

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