Mention in the book that offline docs are installed

[mdinger]

State in installation that offline local docs are installed with Rust and note the location of them. This question comes up often.

[mdinger]

State how to generate a PDF it it's possible unless you want to start bundling a PDF with the installation (which isn't that bad an idea). This is a secondary goal though.

[carols10cents]

It'd also be cool if the end of installation pointed this out as well, something like:

$ sudo ./install.sh 
install: creating uninstall script at /usr/local/lib/rustlib/uninstall.sh
install: installing component 'rustc'
install: installing component 'cargo'
install: installing component 'rust-docs'

    Rust is ready to roll.
    Offline API docs: `open /usr/local/share/doc/rust/html/std/index.html`
    Offline book: `open /usr/local/share/doc/rust/html/book/index.html`

[mdinger]

cc @brson