Missed one backing field.

Still a lot of docs work to follow-up with here.

Lots of cleanup here to make additional bits later more straightforward and less conflicting.

Same order where possible and eliminated the Browsable attribute diffrence with a shim instead.

After 50547eba, TaskSource.Create and TaskSource.CreateDenyExecSync methods are the same. There was some crud left everywhere that appeared like differences but it's really determined once in the static initializer for TaskSource based on the platform anyway.

I've fixed the tests as well - they were a bit backwards on logic/assumptions so hard to read, and have been failing for some time. Now we'd expect ContinueWithExecSync to *not* be hijacked, and everything else to be...that's how the tests are as well.

Note: the #if DEBUG change is because internals are already visible to tests...just simpler.