Docs for SERVER-25725: Running {setFeatureCompatibilityVersion: "3.4"} on a 3.4 primary should kill 3.2 secondaries

Engineering Ticket Description:

Create a v=2 index on the "admin.system.version" collection when the featureCompatibilityVersion is set to 3.4. Versions of MongoDB earlier than 3.4 will fail create a v=2 index during steady-state replication. The index should be removed when the featureCompatibilityVersion is set to 3.2.

In order to address how the implementation of initial sync on the 3.2 branch uses Cloner::copydb(), which automatically "upgrades" (i.e. silently discards) the index version and builds all indexes with the default index version of the node copying the data, the index version of the v=2 index should be returned as a decimal value in the "listIndexes" command response to prevent a 3.2 secondary from performing initial sync from a 3.4 mongod with featureCompatibilityVersion=3.4.