Docs for SERVER-35513: Forbid transactions on inMemory storage engine

Description:

Transactions are not supported on the inMemory storage engine.

Scope of changes:

Impact to other docs outside of this product:

MVP:

Resources:

Engineering Ticket Description:

Since rollback to timestamp doesn't work on inMemory, sharded transactions won't work, so we should disable it on replica sets so we have parity between sharded systems and single replica sets, at least until rollback is fixed on inMemory