Wrong info on the local availability of the documentation

[fhars]

The "Getting Started" respectively "Installation" sections of both version of the book still claim that "The installer also includes a copy of the documentation locally, so you can read it offline. Run rustup doc to open the local documentation in your browser", which is no longer true. This is aggravated by the fact that 'rustup doc' fails silently, see rust-lang/rustup#901

[steveklabnik]

I'm still advocating that we change this default, but you're right that we'll want to be sure it's correct before we publish.

[carols10cents]

rust-lang/rustup#903

[carols10cents]

The default has been changed back, rust docs are included with rustup installations again! Closing this!