Improve Sleep documentation for !Unpin

[Darksonn]

Currently the documentation for Sleep renders like this:

It is not exactly clear from this documentation that Sleep is !Unpin. This PR manually implements the project method on Sleep so that the documentation is improved to this:

I will let it be up to the rest of the team to decide if this is worth the extra unsafe block.

[taiki-e]

I would recommend adding a document explaining that this future is !Unpin, instead of adding unsafe code.

[quasicomputational]

+1 for documenting this and giving explicit instructions on how to pin Sleep. I just got as far as writing a bug report for the loss of a usable Unpin impl between 0.3.6 and 1.0.0, and only realised it was intentional when I was looking for the commit that 'broke' it and realised it was deliberate.

[jyn514]

@Darksonn would it help if rustdoc could tell that __Origin: Unpin is always false and stop showing the blanket impl if so?

[Darksonn]

Yes, that would solve the problem.

[Darksonn]

Closing in favor of #3457.