Added documentation on Stripe_Charge methods.

The way Stripe classes are architectured, there is no way an IDE can understand what class each method returns, which creates inspection issues in our code base.

This PR adds correct documentation to the following methods in `Stripe_Charge`:

- `retrieve()`
- `capture()`
- `refund()`

I'll be happy to do more work on this project if you're willing to merge my pull requests.