Add additional throws comment for generatToken method.

Change-Id: I9cfc589ade8a91040fc9c447740493fd49e392af

Author: huangjiahua

cab-token-generator/java/com/google/auth/credentialaccessboundary/ClientSideCredentialAccessBoundaryFactory.java

@@ -132,7 +132,9 @@ private ClientSideCredentialAccessBoundaryFactory(Builder builder) {
    * 
    * @param accessBoundary
    * @return The Client-Side CAB token in an {@link AccessToken} object
-   * @throws IOException
+   * @throws IOException If an I/O error occurs while refrehsing the source credentials
+   * @throws CelValidationException If the availability condition is an invalid CEL expression
+   * @throws GeneralSecurityException If an error occurs during encryption
    */
   public AccessToken generateToken(CredentialAccessBoundary accessBoundary)
       throws IOException, CelValidationException, GeneralSecurityException {