Clarify that _id key gets stripped from MongoDB index store in S3Store

[Andrew-S-Rosen]

We should note in the documentation (at least) that the "_id" key gets stripped below. This meant I couldn't use "_id" from MongoStore index store when instantiating my S3Store.

https://github.com/materialsproject/maggma/blob/44a251af2df4eed50b5d0e616d6bb845446c17cc/src/maggma/stores/aws.py#L364-L366