Improve documentation on fields.
Improve documentation on fields.

- You can mix incl/excl as long as it is `_id`
- Note unsupported `$` and `$slice` operators of minimongo
- Give an advanced example with reference to mongodb docs.

Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor,Hansoft/meteor