doc: clarify child_process close event

clarify the 'close' event description in the child_process docs.

fixes: #37998

PR-URL: #38181
Fixes: #37998
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/child_process.md

@@ -991,9 +991,11 @@ added: v0.7.7
 * `code` {number} The exit code if the child exited on its own.
 * `signal` {string} The signal by which the child process was terminated.
 
-The `'close'` event is emitted when the stdio streams of a child process have
-been closed. This is distinct from the [`'exit'`][] event, since multiple
-processes might share the same stdio streams.
+The `'close'` event is emitted after a process has ended _and_ the stdio
+streams of a child process have been closed. This is distinct from the
+[`'exit'`][] event, since multiple processes might share the same stdio
+streams. The `'close'` event will always emit after [`'exit'`][] was
+already emitted, or [`'error'`][] if the child failed to spawn.
 
 ```js
 const { spawn } = require('child_process');