doc: specify return type for tty.isatty()

PR-URL: #27154
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>

doc/api/tty.md

@@ -245,6 +245,7 @@ added: v0.5.8
 -->
 
 * `fd` {number} A numeric file descriptor
+* Returns: {boolean}
 
 The `tty.isatty()` method returns `true` if the given `fd` is associated with
 a TTY and `false` if it is not, including whenever `fd` is not a non-negative