fix(--target): RUSTDOCFLAGS=--cap-lints=warn

[zjp-CN]

close #1068

Cargo relies on warning diagnostics to find information, --cap-lints=allow eliminates the warnings, so set the value to warn level to keep it.

[obi1kenobi]

I'm pretty worried about the possibility of something along the lines of -Dwarnings (either in code as a #[deny], or in the CLI, or in some config file) that causes those generated warnings to turn into errors. We added --cap-lints precisely because of -Dwarnings style issues.

I'd be open to merging such a change if you could include tests and test crates that check as many of those cases as possible, so that we know that --cap-lints=warn is sufficient and won't just cause something else to explode. I'd be happy to point you in the right direction for writing those test cases, I just want to first check if you're open to doing that :)

[zjp-CN]

Thanks for reminding me of -Dwarnings...

The fix here is unnecessary if rust-lang/cargo#15036 lands on nightly in a few days. I'll close this PR when it works.

I can submit a testcast that needs --target argument in another PR. But there seems to have one already?

Update: open #1073 for target testing.