Fixes 10088: Simpler nested headings section in accessibility docs

BS3 commit e4a2c35b84b71082d46c21876bd34f39fff1d81d
correct enable selector for button usage in docs

BS3 commit 19c0d2ad94b81d0ef16b91f76cb6830ec99144c2
Fixes 10082: document `.navbar-form`

BS3 commit 2f2ce15e587868cfbde2906560dae8def0b4880d
fixes 9901. Fixes 9928 and 9932: carousel example in IE10

BS3 commits c84374f9def122c31860260ddeb3049a27da6c2c,
4f28fdd22787c2ff96d69d037760db0b7b40648d
a few wording tweaks

BS3 commit 32c03994ee217ee4501146eeb555954d0ba86e9e