Document (and tweak) adjoin

Officially documenting adjoin's existing behavior.

Also:
- optimize 1-fun case to simply alias the function, instead of wrapping it.
- cleanup parallelism's adjoin code.
- Unrelated: remove duplicate `isAssignable` template in parallelism.

Contains indentation changes. Suggest viewing without taking whites into account:
https://github.com/D-Programming-Language/phobos/pull/2008/files?w=1