Rollup merge of #31327 - dirk:dirk/process-child-safety-docs, r=alexcrichton

`Drop` is not implemented for `Child`, so if it goes out of scope in Rust-land and gets deallocated the child process will continue to exist and execute. If users want a guarantee that the process has finished running and exited they must manually use `kill`, `wait`, or `wait_with_output`.

Fixes #31289.

r? @steveklabnik

src/libstd/process.rs

@@ -47,6 +47,16 @@
 ///
 /// assert!(ecode.success());
 /// ```
+///
+/// # Note
+///
+/// Take note that there is no implementation of
+/// [`Drop`](../../core/ops/trait.Drop.html) for child processes, so if you
+/// do not ensure the `Child` has exited then it will continue to run, even
+/// after the `Child` handle to the child process has gone out of scope.
+///
+/// Calling `wait` (or other functions that wrap around it) will make the
+/// parent process wait until the child has actually exited before continuing.
 #[stable(feature = "process", since = "1.0.0")]
 pub struct Child {
     handle: imp::Process,