docs: add documentation for Builder class in IdTokenCredentials

Author: diegomarquezp

oauth2_http/java/com/google/auth/oauth2/IdTokenCredentials.java

@@ -161,6 +161,10 @@ public static Builder newBuilder() {
     return new Builder();
   }
 
+  /**
+   * Builder of IdTokenCredentials.
+   * For usage and examples, see {@link IdTokenCredentials}
+   */
   public static class Builder extends OAuth2Credentials.Builder {
 
     private IdTokenProvider idTokenProvider;