Document html_root_url

[dtolnay]

This is in the first edition but not the second edition.

I was not able to find any authoritative documentation about this important attribute and the API guidelines does not seem like the right place for that (rust-lang/api-guidelines#77).

[steveklabnik]

I am tempted to say that this belongs in rustdoc documentation, rather than in the book.

[dtolnay]

I didn't realize there was rustdoc documentation.

[steveklabnik]

There is not, but there should be / there will be.

[carols10cents]

I've moved this to rust-lang/rust#42322. Thanks!