rustdoc: `--show-coverage --output-format=json` doesn't respect `-o`, but the docs suggest it will.

[aDotInTheVoid]

In the docs for --show-coverage --output-format=json we say:

If you want the JSON output to be displayed on stdout instead of having a file generated, you can
use -o -.

-- https://github.com/rust-lang/rust/blob/f10db292a3733b5c67c8da8c7661195ff4b05774/src/doc/rustdoc/src/unstable-features.md#json-output

This is true!

$ rustdoc +nightly ./lol.rs --show-coverage --output-format=json -Zunstable-options -o -
{"./lol.rs":{"total":2,"with_docs":0,"total_examples":1,"with_examples":0}}

But if you use -o file.json it still prints to stdout:

$ rustdoc +nightly ./lol.rs --show-coverage --output-format=json -Zunstable-options -o file.json
{"./lol.rs":{"total":2,"with_docs":0,"total_examples":1,"with_examples":0}}
$ ls
lol.rs

In fact, rustdoc always prints to stdout, and the -o flag does nothing:

rust/src/librustdoc/passes/calculate_doc_coverage.rs

Lines 132 to 138 in f10db29

	fn print_results(&self) {
	let output_format = self.ctx.output_format;
	// In this case we want to ensure that the `OutputFormat` is JSON and NOT the `DocContext`.
	if output_format.is_json() {
	println!("{}", self.to_json());
	return;
	}

The docs should be updated to be clear about this.

Also, maybe --emit=dep-info shouldn't be supported when using --show-coverage, and there's no "output file" to speak of.

[aDotInTheVoid]

@rustbot modify labels: +T-rustdoc +T-rustdoc-internals +A-docs +A-doc-coverage -needs-triage