rustdoc: Cleanup doc string collapsing

[petrochenkov]

doc_value and (former) collapsed_doc_value can be implemented in terms of each other, and doc_value doesn't need the Option.

This PR doesn't do any semantic changes, only refactoring, although some pre-existing choices between doc_value and collapsed_doc_value across rustdoc may be questionable.

[rustbot]

r? @GuillaumeGomez

(rustbot has picked a reviewer for you, use r? to override)

[rustbot]

Some changes occurred in src/librustdoc/clean/types.rs

cc @camelid

[petrochenkov]

Updated unit tests.
@rustbot ready

[GuillaumeGomez]

Thanks for the improvement!

@bors r+ rollup

[bors]

📌 Commit 4082053 has been approved by GuillaumeGomez

It is now in the queue for this repository.

[bors]

⌛ Testing commit 4082053 with merge cade266...

[bors]

☀️ Test successful - checks-actions
Approved by: GuillaumeGomez
Pushing cade266 to master...

[rust-timer]

Finished benchmarking commit (cade266): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results

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)	2.6%	[2.4%, 2.9%]	2
Regressions ❌ (secondary)	1.9%	[1.9%, 1.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.6%	[2.4%, 2.9%]	2

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: 646.557s -> 647.061s (0.08%)