Adds help messages to mongodb_store_db and mongodb_store_uri

Help messages are based on the webpages below:

http://api.mongodb.org/python/current/api/pymongo/mongo_client.html#pymongo.mongo_client.MongoClient
http://api.mongodb.org/python/current/api/gridfs/

DocImpact

Change-Id: Id95459b3eb2b50703e5793e3bf02c9ea1b142236
Merge "Adds help messages to mongodb_store_db and mongodb_store_uri"