Improve JanusGraph Transaction Logs documentation

[cdegroc]

Definitions of Transaction Log, Triggers, Trigger Log, Write-Ahead or System Transaction Log, Bus ... in https://docs.janusgraph.org/advanced-topics/transaction-log/ and https://docs.janusgraph.org/advanced-topics/janusgraph-bus/ are confusing.

In particular, the definition of a Trigger Log in the latter seems to match the definition of Transaction Log in the former. And a Transaction Log in the latter seems to refer only to JanusGraph's System/write-ahead transaction log (enabled by tx.log-tx). The definition of Triggers in the former is also ambiguous.

We should revisit the janusgraph-bus page, align its terminology with the transaction-log page and potentially merge both pages.

In addition, I think a clear distinction should be made between JanusGraph ("not meant for consumption by the user") transaction log (also mentioned in https://docs.janusgraph.org/operations/recovery/#transaction-failure) and user-defined transaction log.