Removed/improved some outdated comments.
Removed/improved some outdated comments.

- Merging the explicit and non-explicit args versions is impossible, because of the sfinaeability requirement. This would otherwise need an if constexpr in the trailing return type, which is not possible, because it will contain return statements.
- The reason why the invoker is put in a separate (base) class is no longer because merging them in the exposer class would result in a lookup ambiguity, but because of the complete refactoring (for unified call syntax).
- import -> treat (slight improved wording).

erikvalkering/smartref,erikvalkering/smartref,erikvalkering/smartref