[Rule Tuning] Add Unit Test and Note for max_signals

[Mikaayenson]

Summary

There are prebuilt rules with max_signals = 10k and the system_limit = 1000 (slack)

We need to update the setup guide to warn the users that those rules that have max_signals > system_limit will be generating at most system_limit alerts per execution (1000 by default) and system_limit = xpack.alerting.rules.run.alerts.max = 1000 by default; can be increased by the user via Kibana config.

We then need to add a unit test to make sure this warning is added when max_signals > system_limit.

Note: in Serverless, users are not able to adjust the system_limit.