chore: rename docsrs to avoid conflicts (relates #6165)

[udoprog]

Since docsrs is such a popular cfg item in the ecosystem it might be worthwhile to try and avoid accidental activation like with #6165

[Darksonn]

I actually think that it's preferable to keep docsrs. That way, downstream crates can access Windows items in Tokio when generating docs on Linux.

Of course, this means that we should make sure that Tokio compiles under docsrs without docs.