Apply suggestions from code review

Author: bshaffer

src/Credentials/ExternalAccountCredentials.php

@@ -42,7 +42,7 @@
  * is used.
  * When the credential configuration is accepted from an
  * untrusted source, you should validate it before creating this class.
- * [documentation] https://cloud.google.com/docs/authentication/external/externally-sourced-credentials for more details.
+ * @see https://cloud.google.com/docs/authentication/external/externally-sourced-credentials
  */
 class ExternalAccountCredentials implements
     FetchAuthTokenInterface,

src/Credentials/ImpersonatedServiceAccountCredentials.php

@@ -37,7 +37,7 @@
  * is used.
  * When the credential configuration is accepted from an
  * untrusted source, you should validate it before creating this class.
- * [documentation] https://cloud.google.com/docs/authentication/external/externally-sourced-credentials for more details.
+ * @see https://cloud.google.com/docs/authentication/external/externally-sourced-credentials
  */
 class ImpersonatedServiceAccountCredentials extends CredentialsLoader implements
     SignBlobInterface,