Improve handling of rustdoc lints when used with raw doc fragments.

[lolbinarycat]

1. rustdoc::bare_urls no longer outputs incoherent suggestions if source_span_for_markdown_range returns None, instead outputting no suggestion
2. source_span_for_markdown_range has one more heuristic, so it will return None less often.
3. add ui test to make sure we don't emit nonsense suggestions.

fixes #135851

[rustbot]

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[lolbinarycat]

@fmease what module do you want me to move source_span_to_markdown_range into, or should that be left for a followup PR?

[lolbinarycat]

@fmease reminder to look at this.

[lolbinarycat]

@fmease it's been a week, are you going to get to this, or should I reroll reviewers?

[fmease]

@bors try @rust-timer queue

[bors]

⌛ Trying commit c6d7696 with merge 75eefd1...

[bors]

☀️ Try build successful - checks-actions
Build commit: 75eefd1 (75eefd14476b7cd550f9b8b82f8fccfa949bf30e)

[rust-timer]

Finished benchmarking commit (75eefd1): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (secondary -4.0%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.0%	[-5.0%, -3.0%]	2
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 776.227s -> 775.389s (-0.11%)
Artifact size: 365.99 MiB -> 365.98 MiB (-0.00%)

[lolbinarycat]

@fmease all recommended changes added