Document core::io

[brson]

It's an important module, and it has no docs.

[catamorphism]

It has some docs now, but it would be nice to document it better for 0.6

[brson]

Doesn't look like this will happen

[bstrie]

Nominating for Maturity 4.

[graydon]

accepted for production-ready milestone

[emberian]

As it is going away and str::rt::io is documented, closing.