Merge unordered from trunk.

Add `BOOST_NOEXPECT` to:

- Move constructors (when appropriate)
- Destructors
- Iterators

Also, fix some misleading documentation about the containers' move support.


[SVN r85048]
Merge unordered to release. Fixes #8851, #8874

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


[SVN r85388]
Merge release notes + float hash fix. Ref #8822.


[SVN r86210]
Merge Visual C++ 12 fix for Unordered.


[SVN r86551]