Documentation for ActivitySource.CreateActivity/StartActivity missing important information about behavior for default Context

Regarding these documentation pages:
[https://docs.microsoft.com/en-us/dotnet/api/system.diagnostics.activitysource.createactivity](ActivitySource.CreateActivity)
[https://docs.microsoft.com/en-us/dotnet/api/system.diagnostics.activitysource.startactivity](ActivitySource.StartActivity)

When one of the following conditions are true:
- `parentContext` argument is `default`, or
- `parentId` argument is `default`, or
- method does not take either of those two arguments,

then the methods parent the new `Activity` to `Activity.Current`.

This is critical information. Discussion can be found in [this thread](https://github.com/dotnet/runtime/issues/65528#issuecomment-1068855998)

cc @tarekgh