allow line numbers in rustdoc

[mdinger]

In documentation it might be nice to talk about specific line numbers so RustDoc should (perhaps optionally) include them in examples. Here they are being added manually which isn't so nice. In the book, there aren't line numbers either.

[steveklabnik]

Triage: no change.

[steveklabnik]

@rust-lang/rustdoc, do we ever plan on implementing this?

[GuillaumeGomez]

Sounds like a good idea. I'll implement it later on.