TestDrive should say clearly it setups only for the top-level block

[nohwnd]

TestDrive docs should make it clear that test drive is setup just once for the whole top level block, no matter if it is called Describe or Context, and is not setup for every block in the top level block.

Original issue:

The documentation could be clearer about this. (It explicitly says Context block when it's actually any nested Context or Describe block.)

Originally posted by @Axiometry in pester/Pester#1837 (comment)