Add disabled central config as a caveat to lambda docs (#3504)

Author: estolfo

docs/setup-aws-lambda.asciidoc

@@ -88,6 +88,8 @@ Errors related to traced spans will be sent as well.
 ==== Caveats
 * System and custom metrics are not collected for Lambda functions. This is both because most of those are irrelevant
 and because the interval-based event sending model is not suitable for FaaS environments.
+* Central Configuration is disabled, which means that the APM agent configuration cannot be changed without redefining
+the lambda environment variables or APM agent settings.
 * Cold starts can be significantly slower when the agent is installed. If this is an issue, following are ways to deal with slow code
 starts:
 ** If the only issue with slower cold starts is Lambda timing out, consider increasing the configured timeout.