Add deprecation note for invoices/credit endpoint

[vruizext]

invoices/credit_and_retract provides the same functionality and it's easier to implement.

We've been meaning to deprecate it for some time, but were waiting that all partners confirmed the integration of the credit_and_retract. Now that there aren't any partners using #credit in production, time has come to deprecate the #credit endpoint

[vruizext]

We've decided to directly remove documentation for invoices#credit and start its deprecation in the codebase