ExitStatus: Improve documentation re wait status vs exit status

The use of `ExitStatus` as the Rust type name for a Unix *wait
status*, not an *exit status*, is very confusing, but sadly probably
too late to change.

This area is confusing enough in Unix already (and many programmers
are already confuxed).  We can at least document it.

I chose *not* to mention the way shells like to exit with signal
numbers, thus turning signal numbers into exit statuses.  This is only
relevant for Rust programs using `std::process` if they run shells.

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

library/std/src/process.rs

@@ -885,7 +885,7 @@ impl Command {
     }
 
     /// Executes a command as a child process, waiting for it to finish and
-    /// collecting its exit status.
+    /// collecting its status.
     ///
     /// By default, stdin, stdout and stderr are inherited from the parent.
     ///
@@ -899,7 +899,7 @@ impl Command {
     ///                      .status()
     ///                      .expect("failed to execute process");
     ///
-    /// println!("process exited with: {}", status);
+    /// println!("process finished with: {}", status);
     ///
     /// assert!(status.success());
     /// ```
@@ -1368,11 +1368,17 @@ impl From<fs::File> for Stdio {
 
 /// Describes the result of a process after it has terminated.
 ///
-/// This `struct` is used to represent the exit status of a child process.
+/// This `struct` is used to represent the exit status or other termination of a child process.
 /// Child processes are created via the [`Command`] struct and their exit
 /// status is exposed through the [`status`] method, or the [`wait`] method
 /// of a [`Child`] process.
 ///
+/// An `ExitStatus` represents every possible disposition of a process.  On Unix this
+/// is the **wait status**.  It is *not* simply an *exit status* (a value passed to `exit`).
+///
+/// For proper error reporting of failed processes, print the value of `ExitStatus` using its
+/// implementation of [`Display`](crate::fmt::Display).
+///
 /// [`status`]: Command::status
 /// [`wait`]: Child::wait
 #[derive(PartialEq, Eq, Clone, Copy, Debug)]
@@ -1400,7 +1406,7 @@ impl ExitStatus {
     /// if status.success() {
     ///     println!("'projects/' directory created");
     /// } else {
-    ///     println!("failed to create 'projects/' directory");
+    ///     println!("failed to create 'projects/' directory: {}", status);
     /// }
     /// ```
     #[stable(feature = "process", since = "1.0.0")]
@@ -1410,9 +1416,14 @@ impl ExitStatus {
 
     /// Returns the exit code of the process, if any.
     ///
-    /// On Unix, this will return `None` if the process was terminated
-    /// by a signal; `std::os::unix` provides an extension trait for
-    /// extracting the signal and other details from the `ExitStatus`.
+    /// In Unix terms the return value is the **exit status**: the value passed to `exit`, if the
+    /// process finished by calling `exit`.  Note that on Unix the exit status is truncated to 8
+    /// bits, and that values that didn't come from a program's call to `exit` may be invented the
+    /// runtime system (often, for example, 255, 254, 127 or 126).
+    ///
+    /// On Unix, this will return `None` if the process was terminated by a signal.
+    /// [`ExitStatusExt`](crate::os::unix::process::ExitStatusExt) is an
+    /// extension trait for extracting any such signal, and other details, from the `ExitStatus`.
     ///
     /// # Examples
     ///

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

@@ -186,12 +186,20 @@ impl CommandExt for process::Command {
 
 /// Unix-specific extensions to [`process::ExitStatus`].
 ///
+/// On Unix, `ExitStatus` **does not necessarily represent an exit status**, as 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.
+///
+/// This is because a Unix wait status (a Rust `ExitStatus`) can represent a Unix exit status, but
+/// can also represent other kinds of process event.
+///
 /// This trait is sealed: it cannot be implemented outside the standard library.
 /// This is so that future additional methods are not breaking changes.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait ExitStatusExt: Sealed {
-    /// Creates a new `ExitStatus` from the raw underlying `i32` return value of
-    /// a process.
+    /// Creates a new `ExitStatus` from the raw underlying integer status value from `wait`
+    ///
+    /// The value should be a **wait status, not an exit status**.
     #[stable(feature = "exit_status_from", since = "1.12.0")]
     fn from_raw(raw: i32) -> Self;
 
@@ -220,6 +228,8 @@ pub trait ExitStatusExt: Sealed {
     fn continued(&self) -> bool;
 
     /// Returns the underlying raw `wait` status.
+    ///
+    /// The returned integer is a **wait status, not an exit status**.
     #[unstable(feature = "unix_process_wait_more", issue = "80695")]
     fn into_raw(self) -> i32;
 }