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.

maggma/src/maggma/stores/aws.py

Lines 364 to 366 in 44a251a

	# Remove MongoDB _id from search
	if "_id" in search_doc:
	del search_doc["_id"]