RustdocCommand progress (when silence is false) does not respect configured stderr

[suaviloquence]

This is a little more nebulous than I thought - it works when RustdocCommand::silence is true, but when it is false, since we just forward the rustdoc err stream to stderr (actual tty stderr, not GlobalConfig::stderr), we're not currently using the configured stderr for the rustdoc progress stream. We can redirect it using Command::spawn and a thread that acts as a pipe between the child process stderr and our stderr if we want. Do you think a library user would expect this behavior? That is, would they expect the rustdoc progress output to go through the cargo-semver-checks' configured stderr. I'm leaning towards yes, but it will add more complexity to the function because we have to use threads.

Originally posted by @suaviloquence in #737 (comment)

Doesn't require immediate action; just something to be aware of in the future. Would a library user expect this rustdoc progress to be outputted to the configured stderr? Is there a way to do this without a thread that pipes?