mysql.transaction_registry is not documented in KB

mysql.transaction_registry table exists and is used for transaction-precise versioning, but can't be found anywhere in the KB by search. Please document it, specifically the difference between transaction_id and commit_id, and between begin_timestamp and commit_timestamp – they seem to be not as obvious as they sound.

begin_timestamp name appears misleading. In the context of transactions, one would expect that it's time when the transaction was started (implicitly or explicitly via BEGIN / START TRANSACTION), but experimentation shows it's not so. My best guess is that begin_timestamp is when execution of COMMIT started, and commit_timestamp is when execution of COMMIT finished. If this is indeed meant to be so, the naming is quite unfortunate.