Auto merge of #3914 - jwilm:specifying-dependencies-replace-note, r=alexcrichton

Add note about Cargo.lock behavior with [replace]

The Cargo.lock behavior when using [replace] can be misleading if one
does not expect multiple versions of the crate to be included. This note
is intended to assure users that this behavior is expected, and it
suggests a way to verify their `[replace]` override is preferred.

I ran into this issue and ended up bugging @alexcrichton about it. Hopefully including this will reduce the support burden on cargo devs :smile:.