Auto merge of #116057 - RalfJung:io-safety, r=Mark-Simulacrum

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

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.

Author: bors