process::ExitStatus: Discuss exit vs _exit in a comment.

As discussed here
 https://github.com/rust-lang/rust/pull/88300#issuecomment-936097710

I felt this was the best place to put this (rather than next to
ExitStatusExt).  After all, it's a property of the ExitStatus type on
Unix.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

library/std/src/process.rs

@@ -1417,6 +1417,11 @@ fn from(file: fs::File) -> Stdio {
 ///
 /// [`status`]: Command::status
 /// [`wait`]: Child::wait
+//
+// We speak slightly loosely (here and in various other places in the stdlib docs) about `exit`
+// vs `_exit`.  Naming of Unix system calls is not standardised across Unices, so terminology is a
+// matter of convention and tradition.  For clarity we usually speak of `exit`, even when we might
+// mean an underlying system call such as `_exit`.
 #[derive(PartialEq, Eq, Clone, Copy, Debug)]
 #[stable(feature = "process", since = "1.0.0")]
 pub struct ExitStatus(imp::ExitStatus);