Rectified errors in document.txt and paginator.py

**doc/document.txt**:
  - In the documentation, variable name ```user_collection``` is reused in subsequent code as the ```user_col```.
  - This confusion is cleared and error in the documentation is fixed.

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**mongokit/ paginator**:
  - The comment explaining each ```Paginator``` property (at some places) was having same copy-paste/mistyping.
  - So, the appropriate property explanation is added.