Adding more context for invalidUnmarshalError management inside readJSON function. Updating 'Panicking vs returning errors' section.

cmd/api/helpers.go

@@ -115,6 +115,7 @@ func (app *application) readJSON(w http.ResponseWriter, r *http.Request, dst any
 			return errors.New("body must not be empty")
 
 		// A json.InvalidUnmarshalError error will be returned if we pass something that is not a non-nil pointer to Decode(). We catch this and panic, rather than returning an error to our handler.
+		// This is firmly an unexpected error which we shouldn't see under normal operation, and is something that should be picked up in development and tests long before deployment.
 		case errors.As(err, &invalidUnmarshalError):
 			panic(err)