docs(mongo-memory-server-opts): add MMS specific option "portGeneration"

Author: hasezoey

docs/api/interfaces/mongo-memory-server-opts.md

@@ -9,12 +9,16 @@ API Documentation of `MongoMemoryServerOpts`-Interface
 
 ### instance
 
-Typings: `instance?: MemoryServerInstanceOpts`
+Typings: `instance?: MemoryServerInstanceOpts & ExtraOptions`
 
 Set custom options based on [`MongoMemoryInstanceOpts`](./mongo-memory-instance-opts.md), but ignores some properties:
 
 - `auth` is ignored because it is set via [auth](#auth) property.
 
+Extra options specific to [`MongoMemoryServer` class](../classes/mongo-memory-server.md):
+
+- `portGeneration`: enable / disable port generation, enabled by default.
+
 ### binary
 
 Typings: `binary?: MongoBinaryOpts`