Auto merge of #85746 - m-ou-se:io-error-other, r=joshtriplett

Redefine `ErrorKind::Other` and stop using it in std.

This implements the idea I shared yesterday in the libs meeting when we were discussing how to handle adding new `ErrorKind`s to the standard library: This redefines `Other` to be for *user defined errors only*, and changes all uses of `Other` in the standard library to a `#[doc(hidden)]` and permanently `#[unstable]` `ErrorKind` that users can not match on. This ensures that adding `ErrorKind`s at a later point in time is not a breaking change, since the user couldn't match on these errors anyway. This way, we use the `#[non_exhaustive]` property of the enum in a more effective way.

Open questions:
- How do we check this change doesn't cause too much breakage? Will a crate run help and be enough?
- How do we ensure we don't accidentally start using `Other` again in the standard library? We don't have a `pub(not crate)` or `#[deprecated(in this crate only)]`.

cc https://github.com/rust-lang/rust/pull/79965

cc `@rust-lang/libs` `@ijackson`

r? `@dtolnay`

library/std/src/process.rs

@@ -1658,8 +1658,7 @@ impl Child {
     /// Forces the child process to exit. If the child has already exited, an [`InvalidInput`]
     /// error is returned.
     ///
-    /// The mapping to [`ErrorKind`]s is not part of the compatibility contract of the function,
-    /// especially the [`Other`] kind might change to more specific kinds in the future.
+    /// The mapping to [`ErrorKind`]s is not part of the compatibility contract of the function.
     ///
     /// This is equivalent to sending a SIGKILL on Unix platforms.
     ///
@@ -1680,7 +1679,6 @@ impl Child {
     ///
     /// [`ErrorKind`]: io::ErrorKind
     /// [`InvalidInput`]: io::ErrorKind::InvalidInput
-    /// [`Other`]: io::ErrorKind::Other
     #[stable(feature = "process", since = "1.0.0")]
     pub fn kill(&mut self) -> io::Result<()> {
         self.handle.kill()