Update the documentation for the create command and the createCollection() shell helper to say that they take a new optional parameter called indexOptionDefaults. The only supported subkey is "storageEngine", which should be documented identically to how the "storageEngine" option is documented for the createIndex() shell helper. Perhaps something along the lines of the following.

Optional. Allows users to specify a default configuration to the storage engine for indexes created on a collection. The value of the storageEngine option should take the following form:

{ <storage-engine-name>: <options> }

Storage engine configuration specified when creating indexes are validated and logged to the oplog during replication to support replica sets with members that use different storage engines.

New in version 3.2.