Update generations and update comments for altered distributed system keyspaces

In 4.0, we set memtable_flush_period_in_ms to 0 (from 1 hour previously), and changed default chunk length from 64KiB to 16KiB. On startup we would try to override existing schema definitions for system_traces, system_distributed, and system_auth to reflect these changes, but because these are changes to smaller values, while timestamps remain the same, these will never override existing, pre-upgrade values from 3.0 or 3.11. As a result, a schema migration request will be pushed by a node on every bounce to every replica, causing unnecessary and ultimately fruitless work.

Bumping the generations fixes the issue, and documenting the changes doesn't hurt either.