DOC: Clarified documentation for convert_dates and use_default_dates params
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...
Great thanks @amilbourne 