Document that test mode does not log transactions

Problem/Motivation

Taxjar's sandbox API does not store transactions, it's only intended for confirming the API's responses. As such, there isn't any way of doing end-to-end testing of the Taxjar system with the test/sandbox mode.

Steps to reproduce

Put the taxjar module into test mode, note that transactions are not recorded in the Taxjar sandbox account.

Proposed resolution

Document that the test/sandbox mode does not store transactions, that they will not be visible on the TaxJar dashboard.

Remaining tasks

Add documentation about the problem.
Work out a reasonable way of indicating this in the UI.

User interface changes

The settings page will indicate that the test/sandbox mode will not store transactions in the TaxJar system, thus they will not be available in the transactions dashboard.

API changes

n/a

Data model changes

n/a