Docs for SERVER-38165: Test that prepared transactions work with an in-memory secondary

Description

Description:

This commit activated testing of transactions on the inMemory storage engine, which was successful.

Engineering Ticket Description:

SERVER-37943 makes transactions work with the in-memory storage engine. In 4.0 an in-memory secondary can apply transactions, an in-memory primary just won't accept them. Even if we do not make transactions work on in-memory primaries, they must work for in-memory secondaries.

Scope of changes

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MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)