chore: move warning to ConnectionProperties

Author: diegomarquezp

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

@@ -270,7 +270,13 @@ public class ConnectionProperties {
           ENCODED_CREDENTIALS_PROPERTY_NAME,
           "Base64-encoded credentials to use for this connection. If neither this property or a"
               + " credentials location are set, the connection will use the default Google Cloud"
-              + " credentials for the runtime environment.",
+              + " credentials for the runtime environment."
+              + " 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",
           null,
           StringValueConverter.INSTANCE,
           Context.STARTUP);