Docs for SERVER-23476: Re-enable resync command for replica set nodes

Engineering Ticket Description:

Since we have temporarily disabled the resync command while reworking code and bringing the DataReplicator into the mix, we need to re-enable support for it and its tests.

These are the expected behaviors:

• Each resync command will cancel any existing data replication and start a new resync (an initial sync)
• A resync will block waiting for the initial sync to complete
• During an initial sync the ReplSetSyncFrom command will set the sync source and issue a resync command internally – having the effect that the initial syncs first attempt uses the supplied host.