Updated default value of the uploadfolder

Per gluon/dal.py (line 9882) in Web2py 2.9.5 - The default value of the uploadfolder is None.
Updated the documentation to reflect the same.
Additional note to default uploadsfolder behavior

Based on suggestions from anthony, clarified that even if default uploadsfolder is None, it doesn't impact most DB adapters.
Merge pull request #192 from mandarvaze/patch-1

Updated default value of the uploadfolder