Docs for SERVER-40156: Initial implementation of Split Horizon

Description

Description:

Need to document the `horizons` field of member config, and the semantics thereof.

Engineering Ticket Description:

Refine and land the PoC that was used to answer questions during the design phase.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

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