Specify that the In-Memory Storage Engine is MongoDB Enterprise only

InMemory is only available in Enterprise as of 3.4 :
https://groups.google.com/forum/#!topic/mongodb-user/ATSJKO6_m9s

The docs page should possibly have a warning banner about this as the mongoD error treats it as a regular unrecognized command

Unrecognized option: storage.inMemory.engineConfig.inMemorySizeGB

try 'mongod --help' for more information