[library/std/src/process.rs] Update docstring with @joshtriplett's re…

…placement text

std/src/process.rs

@@ -1937,10 +1937,14 @@ impl crate::error::Error for ExitStatusError {}
 /// to its parent under normal termination.
 ///
 /// `ExitCode` is intended to be consumed only by the standard library (via
-/// [`Termination::report()`]), and intentionally does not provide accessors like
-/// `PartialEq`, `Eq`, or `Hash`. Instead the standard library provides the
-/// canonical `SUCCESS` and `FAILURE` exit codes as well as `From<u8> for
-/// ExitCode` for constructing other arbitrary exit codes.
+/// [`Termination::report()`]). For forwards compatibility with potentially
+/// unusual targets, this type currently does not provide `Eq`, `Hash`, or
+/// access to the raw value. This type does provide `PartialEq` for
+/// comparison, but note that there may potentially be multiple failure
+/// codes, some of which will _not_ compare equal to `ExitCode::FAILURE`.
+/// The standard library provides the canonical `SUCCESS` and `FAILURE`
+/// exit codes as well as `From<u8> for ExitCode` for constructing other
+/// arbitrary exit codes.
 ///
 /// # Portability
 ///