Auto merge of #5713 - mbrubeck:doc, r=alexcrichton

Auto merge of #5713 - mbrubeck:doc, r=alexcrichton

Clarify FAQ about Cargo.lock for libraries

The current wording in the FAQ implies that checking in a Cargo.lock for a library could cause version conflicts in consumers of the library, which is not true.

The text in the FAQ was based on [this comment][1] (emphasis mine):

> If each dependent library checked in a Cargo.lock, **and Cargo used it,** you would instead get multiple, duplicate copies of those dependencies

but it accidentally dropped the "and Cargo used it" requirement, which I think is the more important part.

[1]: https://github.com/rust-lang/cargo/pull/534#issuecomment-54776481