Fix false positive in suspicious_open_options and make paths work

[y21]

Implements the last few things I had in mind for rust-clippy#11608

This already looked pretty good. The fix for the false positive I had mentioned was basically 90% done. Only the matching needed to be fixed and some paths were incorrect.

Summarising my changes:

• A few touch ups in the documentation, like adding inline codeblocks for code snippets and adding linebreaks where the text was a bit too "dense"
• Making the documentation example no_run. At some point in the last months (after this branched off master) a change was made that changed the default test mode to no_run, because almost all of the doc tests don't need to be run (only compiled).
• Updating paths (and reordering them a bit in alphabetic order).
  • This was the reason why it didn't detect tokio items. The re-exported path is tokio::fs::OpenOptions, but clippy's path methods compare them to the path where they're defined. The real definition path is tokio::fs::open_options::OpenOptions.
• Bail on unknown trait method calls when collecting options (see the last example for a potential false positive)
• Make the logic checking of the final OpenOptions::new() call in the chain work
• Splitting up the help messages, so that there's only one sentence per help

---

Regarding

In particular, we should be able to eventually uncomment the expected error line in tests/ui/open_options.rs:32

This is a case where it's ok to lint, but we currently aren't because it requires tracking calls across statements. I think it's doable, but imo we could/should land it as is with that false negative and fix that as a followup.
In particular, we'd need to be careful here because of control flow. For example:

options.read(true);
if condition {
  options.read(false);
}

... shouldn't be linted since that conditionally overwrites it (and a few other cases).

[y21]

( @atwam I don't know if you get notifications on this repo so I'm pinging you, just in case so it doesn't go unnoticed :) )

[atwam]

Excellent, thanks for cleaning up my code and fixing the remaining issues with it!.