expand documentation for readline.emitKeypressEvents

`readline.emitKeypressEvents` needs `stream` to be in raw mode, ref #6626

Author: arve0

doc/api/readline.md

@@ -362,6 +362,15 @@ Move cursor to the specified position in a given TTY stream.
 Causes `stream` to begin emitting `'keypress'` events corresponding to its
 input.
 
+Note that the stream, if it is a TTY, needs to be in raw mode:
+```js
+readline.emitKeypressEvents(process.stdin);
+if (process.stdin.isTTY) {
+  // might not be a TTY if spawned from another node process
+  process.stdin.setRawMode(true);
+}
+```
+
 ## readline.moveCursor(stream, dx, dy)
 
 Move cursor relative to it's current position in a given TTY stream.