HADOOP-148: Fixes documentation for Pig and various minor inconsistencies

See [HADOOP-148](https://jira.mongodb.org/browse/HADOOP-148) for details.
Because MongoLoader takes `schema` and `idAlias` in a different order than `BSONLoader`. It happened twice to us already that we were switching between the two and forgot to change the order of constructor parameters. As mentioned in [HADOOP-148](https://jira.mongodb.org/browse/HADOOP-148) I think it's worth to be consistent here even though it's a backwards-incompatible change.