fix OS-specific I/O safety docs since the io_safety feature is stable

[RalfJung]

Looks like this text was forgotten to be updated when io_safety got stabilized: it still says "once the io_safety feature is stable".

Also adjust the wording a bit for how these docs relate to the general concept of I/O safety.

[rustbot]

r? @Mark-Simulacrum

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[Mark-Simulacrum]

@bors r+

[bors]

📌 Commit 813fed2 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit 813fed2 with merge 5105b1e...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 5105b1e to master...

[rust-timer]

Finished benchmarking commit (5105b1e): 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)	3.1%	[1.4%, 4.3%]	4
Regressions ❌ (secondary)	3.1%	[3.1%, 3.1%]	1
Improvements ✅ (primary)	-2.6%	[-2.6%, -2.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.0%	[-2.6%, 4.3%]	5

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: 631.523s -> 631.647s (0.02%)
Artifact size: 317.27 MiB -> 317.29 MiB (0.01%)