unix::ExitStatus: Add comment saying that it's a wait status

With cross-reference.

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

library/std/src/sys/unix/process/process_unix.rs

@@ -616,6 +616,9 @@ impl Process {
 }
 
 /// Unix exit statuses
+//
+// This is not actually an "exit status" in Unix terminology.  Rather, it is a "wait status".
+// See the discussion in comments and doc comments for `std::process::ExitStatus`.
 #[derive(PartialEq, Eq, Clone, Copy)]
 pub struct ExitStatus(c_int);