chore(docs): update docstrings for batches without context managers

[daniel-sanche]

Fixes #447

The library uses context managers to make transaction/batch lifecycle management easier. When in a context manager, all client.get calls will target the active transaciton, and commit/rollback is called automatically when complete

You can also manage the lifecycle explicitly, but you have to manually pass in the transaction when doing client.get calls to avoid contention exceptions.

This PR updates the docstrings to make the contention issue more clear, and adds a test using transactions without context managers