use .col-xs-* in "Forms: Column Sizing" docs so more responsive. table contextual classes docs: .col-lg-* => .col-xs-*. Remove "Remove margin on the body" from base css docs

BS3 commits: cc3f09d4d1b385d578c9b77956e11f568e98dd2d,
667002657259b4b29e85a28f38d3be25cd21f778,
25954702ea7eb9d7e0596159041731f9b1fe464e