doc: mark optional parameters in timers.md

PR-URL: #35764
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

doc/api/timers.md

@@ -188,14 +188,14 @@ async function timerExample() {
 timerExample();
 ```
 
-### `setInterval(callback, delay[, ...args])`
+### `setInterval(callback[, delay[, ...args]])`
 <!-- YAML
 added: v0.0.1
 -->
 
 * `callback` {Function} The function to call when the timer elapses.
 * `delay` {number} The number of milliseconds to wait before calling the
-  `callback`.
+  `callback`. **Default**: `1`.
 * `...args` {any} Optional arguments to pass when the `callback` is called.
 * Returns: {Timeout} for use with [`clearInterval()`][]
 
@@ -206,14 +206,14 @@ set to `1`. Non-integer delays are truncated to an integer.
 
 If `callback` is not a function, a [`TypeError`][] will be thrown.
 
-### `setTimeout(callback, delay[, ...args])`
+### `setTimeout(callback[, delay[, ...args]])`
 <!-- YAML
 added: v0.0.1
 -->
 
 * `callback` {Function} The function to call when the timer elapses.
 * `delay` {number} The number of milliseconds to wait before calling the
-  `callback`.
+  `callback`. **Default**: `1`.
 * `...args` {any} Optional arguments to pass when the `callback` is called.
 * Returns: {Timeout} for use with [`clearTimeout()`][]