[Observability Alerts] Update observability alerts setup section

[mohamedhamed-ahmed]

Summary

During the implementation of some alert privilege checks in Rule creations, it was found out that Custom Threshold Rule can be created if the user has Stack Alerts Kibana feature privilege.

This however doesn't match with the current documentation where it mentions that
For Observability rules, you must have all privileges for the appropriate Observability features.

What is needed

We should ideally update the docs so that its clear that even with only Stack Alerts feature privilege the user will be able to create Custom Threshold Rule.