Update read preference documentation to include impact of elections

Description

In our documentation for read preference, we should include:

Primary: if a query is running on a primary node and there is an election, the query will run to completion after the step down.

Secondary:  if a query is running on a secondary node and there is an election, the query will run to completion after the step up.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

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