doc: clarify tty.isRaw

PR-URL: #48055
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

Author: jrvidal

doc/api/tty.md

@@ -51,7 +51,11 @@ added: v0.7.7
 -->
 
 A `boolean` that is `true` if the TTY is currently configured to operate as a
-raw device. Defaults to `false`.
+raw device.
+
+This flag is always `false` when a process starts, even if the terminal is
+operating in raw mode. Its value will change with subsequent calls to
+`setRawMode`.
 
 ### `readStream.isTTY`