Auto merge of #9121 - vext01:extra-link-arg-doc, r=alexcrichton

Document -Z extra-link-arg.

Whilst reading the cargo source code, I encountered `-Z extra-link-arg`. I'd missed this flag before because it is not documented in `-Z help`.

This commit adds the one-liner documentation.

([This argument **is** mentioned in the unstable cargo book](https://doc.rust-lang.org/beta/cargo/reference/unstable.html#extra-link-arg))