Document programmatic tx mgt support in the TCF

This commit introduces a new "Programmatic transaction management"
section in the Testing chapter of the reference manual with an example
highlighting the new support provided via TestTransaction.

In addition, this commit begins the work to be addressed more
thoroughly in SPR-11399 by overhauling the entire "Transaction
management" section with in-depth discussions on the following topics.

 - Test-managed transactions
 - Enabling and disabling transactions
 - Transaction rollback and commit behavior
 - Executing code outside of a transaction
 - Configuring a transaction manager

Issue: SPR-11941, SPR-11399