Docs for SERVER-39989: Use a config.transactions find command for the begin fetching timestamp in initial sync

Description

Description:

The last commit removed oldestActiveTransactionOpTime from serverStatus.transactions. If that field was ever documented, that should be removed.

Engineering Ticket Description:

Initial sync will do a readConcern: local find on the config.transactions table instead of calling serverStatus after the changes in SERVER-39680 as described in SERVER-36494.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

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