chore(docs): add warning for encoded credential

Author: diegomarquezp

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/ConnectionOptions.java

@@ -229,6 +229,14 @@ public class ConnectionOptions {
   /** Name of the 'encodedCredentials' connection property. */
   public static final String ENCODED_CREDENTIALS_PROPERTY_NAME = "encodedCredentials";
 
+  /** System property used to enable encoded credentials.
+   * WARNING: Enabling this property without proper validation can
+   * expose the application to security risks.
+   * It is intended for use with credentials from a trusted source only,
+   * as it could otherwise allow end-users to supply arbitrary credentials.
+   * For more information, see
+   * https://cloud.google.com/docs/authentication/client-libraries#external-credentials
+   */
   public static final String ENABLE_ENCODED_CREDENTIALS_SYSTEM_PROPERTY =
       "ENABLE_ENCODED_CREDENTIALS";