Unix ExitStatus comments and a tiny docs fix

library/std/src/os/unix/process.rs

@@ -207,7 +207,7 @@ impl CommandExt for process::Command {
 /// [`ExitStatusError`](process::ExitStatusError).
 ///
 /// On Unix, `ExitStatus` **does not necessarily represent an exit status**, as
-/// passed to the `exit` system call or returned by
+/// passed to the `_exit` system call or returned by
 /// [`ExitStatus::code()`](crate::process::ExitStatus::code).  It represents **any wait status**
 /// as returned by one of the `wait` family of system
 /// calls.

library/std/src/process.rs

@@ -1417,6 +1417,11 @@ impl From<fs::File> for 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);

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);