Replacing JRegistry with Joomla\Registry\Registry in some docblocs

### Issue
Currently, IDEs like PHPStorm are showing errors for JRegistry objects because it can't find the class. This is actually correct because JRegistry doesn't exist anymore. It's now a namespaced class from the framework called `Joomla\Registry\Registry`.
You can still use `new JRegistry` to create a Registry object because we have some alias set up in our loader.

### Proposed Solution
As a first step I change the doc blocs so they point to the real class instead of the alias. Meaning I changed `JRegistry` to `Joomla\Registry\Registry`. That should allow IDEs to find it at least in some instances. Probably not in all places.
As I only change doc blocs, it should have no impact on the actual code at all.

### Testing
Test with your IDE if you get less warnings about undefined JRegistry class and its methods.
In the actual CMS, nothing should change at all.

### Future steps
There are still over 800 uses of `JRegistry` in the core code. I think creating new Registry objects would be easy to change as well to `new Joomla\Registry\Registry`.
I'm not yet sure how the type hinting has to be done. Maybe anyone already used that with namespaced objects?

Feedback welcome!