Rollup merge of #106964 - workingjubilee:crouching-ioerror-hidden-documentation, r=ChrisDenton

Clarify `Error::last_os_error` can be weird

Fundamentally, querying the OS for error codes is a process that is deeply subject to the whims of chance and fortune. We can account for OS, but not for every combination of platform APIs. A compiled binary may not recognize new errors introduced years later. We should clarify a few especially odd situations, and what they mean: We can effectively promise nothing... if you ask for Rust to decode errors where none have occurred.

This allows removing mention of ErrorKind::Uncategorized.
That error variant is hidden deliberately, so we should not explicitly mention it.

This fixes #106937.

Since you had an opinion also: Does this solution seem acceptable?
r? ``@ChrisDenton``

Author: matthiaskrgr