Update Callback type hint

Author: risantos

src/types/index.d.ts

@@ -60,7 +60,10 @@ export interface ValidatorJSAsserts {
   /** Valid Canadian ZIP code (postal code). */
   caZipCode(): AssertInstance;
 
-  /** Run a custom callback function, passing a custom class name. */
+  /**
+   * Run a custom callback function, passing a custom class name.
+   * - Class names must match the pattern `/^[a-zA-Z\d]+$/`.
+   */
   callback(fn: (value: unknown) => boolean, customClass: string): AssertInstance;
 
   /** Valid Brazilian CPF number. @requires cpf */