Add ts_ms field to examples of transaction boundary events and examples and update property description in documentation

Applies to MySQL, Oracle, PG, and Vitess connectors.

The transaction boundary documentation includes a description for the ts_ms field, but the associated examples don't show the ts_ms field.
Should I update the examples to include the field?

The ts_ms field is shown in the Change data event enrichment examples for all of the connectors (It's currently called Data change event enrichment in Db2):

To align with other instances where we mention a ts_ms field, such as in the context of create or update events, update the current description of the ts_ms field:

From: "The time of BEGIN or END transaction"..

To: "The time at which the beginning or end of a transaction occurs, if this information is provided by data source. If the data source does not provide the transaction time, then the field represents the time at which Debezium processed the transaction.