There are two sentences on http://docs.mongodb.org/manual/core/index-single/ that could use clarification or rewording:
1 - "MongoDB supports indexes that contain either a single field or multiple fields depending on the operations that index supports."
--> This sentence is a bit unclear because it's using 'index' to refer to index types and individual indexes. Perhaps it should say 'depending on the operations that index type supports' or something similar?
2 - "Think of the _id field as the primary key for a collection. Every document must have a unique _id field. You may store any unique value in the _id field. The default value of _id is an ObjectID on every insert() operation. An ObjectId is a 12-byte unique identifiers suitable for use as the value of an _id field."
--> This sentence is hard to parse. It's also a bit misleading because save() can automatically generate an ObjectId, as well.