In WT-9881 the flush_tier configuration setting was added to checkpoint. It brings up the question about what the system should do if someone calls checkpoint with session->checkpoint("flush_tier=(enabled)") when tiered storage is not being used.

In the current code that performs a checkpoint and succeeds. We should reflect that in the documentation to say:

if true and tiered storage is in use, perform one iteration of object switching and 
flushing objects to tiered storage

Essentially it is a no-op to enable it when not using tiered storage.