Fix rustdoc item summaries that are headers

[chrisduerr]

Rustoc item summaries that are headers were not displayed at all because
they started with whitespace.

This PR fixes this and now removes the whitespace and then displays the
block.

I'm not sure if the rustdoc test is written correctly, if there's anything to improve, just let me know. :)

This fixes #46377.

This is how it looks when rendered out now:

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @steveklabnik (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[QuietMisdreavus]

Thanks so much for writing this! r=me if Travis goes green.

[QuietMisdreavus]

@bors r+

All looks good to me! Travis checked out, so it looks like the test passes. Thanks again!

[bors]

📌 Commit 91a4106 has been approved by QuietMisdreavus

[chrisduerr]

Thanks for walking me through this! :)

[QuietMisdreavus]

There was some mishaps with commits here, so let me make sure bors sees this PR correctly...

@bors r+

[bors]

📌 Commit 91a4106 has been approved by QuietMisdreavus

[frewsxcv]

@bors rollup