Explanation of AddFilter Func string params

[mrek-msft]

For users of Microsoft.Extensions.Logging, it is not clear what string? parameter means when implementing custom filtering rules using FilterLoggingBuilderExtensions.AddFilter:

It would be nice to have some better comment on these methods. Docs here for this particular overload also do not tell much.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-extensions-logging
See info in area-owners.md if you want to be subscribed.