Can we have some more info on Send and Sync

[montekki]

The current documentation has a TODO: better explain what can or can't be Send or Sync. Sufficient to appeal only to data races?

The explanation on what can and can't be Send or Sync would be very useful. For instance, it could contain why LinkedList is Send while it contains pointer fields that are not Send and this is Ok and the changes that could be introduced to this container so it actually stops being Send.

Thanks