Merge unordered from trunk.

Add `BOOST_NOEXPECT` to:

- Move constructors (when appropriate)
- Destructors
- Iterators

Also, fix some misleading documentation about the containers' move support.
Merge unordered to release. Fixes #8851, #8874

Avoid some warnings, and move detail functions into a sub-namespace to avoid
exposing them via. ADL.