rustdoc: use .rustdoc class instead of body

[notriddle]

This didn't show up in our local tests, because the problem is actually caused by docs.rs rewritten HTML (which relocates the classes that this code looked for from the body tag to a child div).

Fixes #117290

r? @GuillaumeGomez

Both problems are regressions introduced by #115948

[rustbot]

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @jsha

[GuillaumeGomez]

Thanks!

@bors r+ rollup

[bors]

📌 Commit 8b1bcc8 has been approved by GuillaumeGomez

It is now in the queue for this repository.

[bors]

⌛ Testing commit 8b1bcc8 with merge 891c6ee...

[bors]

☀️ Test successful - checks-actions
Approved by: GuillaumeGomez
Pushing 891c6ee to master...

[rust-timer]

Finished benchmarking commit (891c6ee): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was 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)	-0.3%	[-0.3%, -0.3%]	1
All ❌✅ (primary)	-	-	0

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)	2.0%	[1.1%, 2.9%]	2
Regressions ❌ (secondary)	4.0%	[4.0%, 4.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.0%	[1.1%, 2.9%]	2

Cycles

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

Binary size

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

Bootstrap: 674.132s -> 674.362s (0.03%)
Artifact size: 311.12 MiB -> 311.11 MiB (-0.00%)