Remove Real trait and move methods into Float

This is part of the effort to simplify `std::num`, as tracked in issue #10387.
Removing '15.3 Do syntax'since it is deprecated.

The 'do' keyword was deprecated in 0.10, and is keep as
reserved keyword.

So the tutorial part about doesn't make sense.
The spawning explanation was move into '15.2 Closure compatibility'.

Fixing misspelling.

Thanks for precisions.

Moved from 15.2 to 15.1.

Fixed typo, and apply pnkfelix advices.