Document the removal of FS default queue creation

In YARN-7769 the auto creation of the root.default queue is removed from FS initialization. This impacts the the documentation in multiple places:

• By default, all users share a single queue, named “default”.
• user-as-default-queue says it falls back to the default queue
• allow-undeclared-pools is another point that mentions the default queue

This should be changed according to the new state.