DOC: Clarified documentation for convert_dates and use_default_dates params

[amilbourne]

This hopefully clarifies the documentation for the behaviour of the convert_dates and keep_default_dates parameters on the read_json function.

I recently had to use this function and found the existing docs a bit confusing. I actually created a truth table (below), but this is probably a bit much for the docs.

convert_dates	keep_default_dates	Result
True	True	Convert default date fields
True	False	Don’t convert anything
False	True	Don’t convert anything
False	False	Don’t convert anything
List	True	Convert listed and default date fields
List	False	Convert just listed date fields

I hope you agree that my changes make the behaviour clearer - but perhaps people will disagree...

[WillAyd]

minor comment otherwise lgtm

[WillAyd]

Great thanks @amilbourne