I'm not sure I completely understand the question, but I'll give it a shot anyway....
Just to be clear starting out: "documents" in MongoDB are actually JSON-like objects, not text documents like you'd maybe use with Microsoft
Word or other word processors.
As to how you can do text analytics on fields within MongoDB documents, you can use the recently released
full text search to do searching, and there are a limited number of string-processing functions within the
aggregation framework, but in general I'd say that text processing isn't the best use case for MongoDB.
If you decide to do it anyway, you could certainly use MongoDB as the persistence layer of your analytics engine, but it's not going to provide you with many benefits over any other persistence engine.
Hope that helps!