adjust description & give source for nested `.btn-group`s example
adjust description & give source for nested `.btn-group`s example

BS3 commit 509ffe110ecf78998a32e5603abedc79d526235d

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