Docs for SERVER-38143: 4.2 -- remove mmapv1 command-line options/config file settings?

Description

Description:

• The command-line option --noprealloc should go away as it was for mmapv1.

• replication.secondaryIndexPrefetch and --replIndexPrefetch have also been removed in this patch set as they were for mmapv1.

Engineering Ticket Description:

• The mongod still has the command-line option --noprealloc . Should this just go away?

• Also, we documented that replication.secondaryIndexPrefetch and --replIndexPrefetch are mmapv1 (since a while now). Should they also be gone? Or do we need to update our docs?

Scope of changes

Impact to Other Docs

MVP (Work and Date)

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