Auto merge of #3063 - Wilfred:patch-1, r=alexcrichton

Auto merge of #3063 - Wilfred:patch-1, r=alexcrichton

Clarify wording: this restriction is not just libraries

I found the previous wording a little confusing (see [discussion on users.rust-lang.org](https://users.rust-lang.org/t/uploading-binary-packages-not-libraries-to-crates-io/7072)).

Since the section heading specifically refers to libraries, I was puzzled that crates.io rejected my binary package with wildcard dependencies (plus a Cargo.lock file). I think this wording is clearer, let me know what you think.

Ideally we'd also say the reasoning behind rejecting wildcard dependencies for binary crates, but I don't know what the reasoning is.