Fix backup documentation to be more accurate with respect to named checkpoints

Our backup documentation says:

In cases where the backup is desired for a checkpoint other than the most recent, applications can discard all checkpoints subsequent to the checkpoint they want using the WT_SESSION::checkpoint method with drop parameter.

That is specific to using named checkpoints, and relies on checkpoints being taken at a specific point in time (not the current approximate checkpoints which rely on content being made durable in the history store).