DOC : added note about maintain ref to widgets

Added note to widget doc-strings that the user must maintain a reference
to the widgets to keep them responsive (if they get gc'd the call backs
go away).

Some random conversion to numpyboc

Closes #3105
DOC : improved docs in widgets.py

 - corrected docstring in Slider.__init__
 - feedback on previous commit
Merge pull request #3132 from tacaswell/widget_doc

DOC : added note about maintain ref to widgets