docs: enhance documentation for _generateEventId function

packages/auth/src/core/util/event_id.ts

@@ -15,6 +15,25 @@
  * limitations under the License.
  */
 
+/**
+ * Generates a unique event identifier with a customizable prefix and length.
+ * 
+ * This function creates a random alphanumeric string by repeatedly generating
+ * random numbers and concatenating their string representations. The resulting
+ * event ID consists of the provided prefix followed by a randomly generated
+ * string of the specified number of digits.
+ * 
+ * @param prefix - The string to prepend to the generated random number sequence. Defaults to an empty string.
+ * @param digits - The number of random digits to generate for the event ID. Defaults to 10.
+ * @returns A string containing the prefix followed by the randomly generated digits.
+ * 
+ * @example
+ * ```typescript
+ * _generateEventId('evt_', 8); // Returns something like: 'evt_12345678'
+ * _generateEventId(); // Returns something like: '1234567890'
+ * ```
+ */
+
 export function _generateEventId(prefix = '', digits = 10): string {
   let random = '';
   for (let i = 0; i < digits; i++) {