Docs for SERVER-35656: Ensure a 4.0 mongos crashes upon attempting to connect to an FCV 4.2 cluster

Description

Description:

Document the changed behavior in adding a 4.0 mongos to a 4.2 cluster

Engineering Ticket Description:

As part of SERVER-32635, a last-stable binary version mongos will crash upon attempting to connect to a latest featureCompatibilityVersion cluster. However, since this behavior was added for 4.0 mongos and FCV 4.2 was not present at the time, some special testing behavior was necessary to simulate a 4.0 mongos crashing. As part of the 4.2 upgrade/downgrade work, the crash_mongos_against_upgraded_cluster.js test should be updated to no longer rely on these failpoints and just directly use a 4.0 mongos and FCV 4.2 cluster. The failpoints/wireVersion added as part of SERVER-32635 solely for simulation of the crash should also be removed.

Scope of changes

• Add note to:
  • mongos ref and conceptual pages
  • sharded cluster update page

• the 4.0 version already mentions the fcv/mongos in 4.0 compatibility:
  • for now, won't add to 4.0 mongos ref and conceptual pages

Impact to Other Docs

MVP (Work and Date)

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