Explain how to configure filters for structured logs

[jamescrosswell]

See this thread below... this really should be in our docs as I don't thing it's obvious for SDK users (it's a slightly different mechanism to the one we have for events and breadcrumbs).

Thank you for taking the time to explain this, and hopefully, this will help others who search for this same issue in the future.

Originally posted by @Mike-E-angelo in #4938

[linear]

NET-593

[Flash0ver]

While at it, we may also clarify our documentation around the difference in filtering for Breadcrumbs/Events and Structured Logs.
See also #3847.