Clarify the 3scale Error Codes section

The documentation page https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.4/html-single/api_devops/index#threescale_error_codes provides a list of status codes and in what conditions they are returned.
However, some clarifications are needed:

1. It is not clear that these codes are what Service Management API returns, as it says "returned by 3scale", and this could also mean "3scale gateway".
A clarification is needed that these codes are the ones returned by Service Management API.

2. The last paragraph says:

Note that, if using the standard API gateway configuration, any non-200 return code from 3scale will result in a 403 being returned to the client.

This is not correct, as the gateway can also return 429 (when limits are reached) and 404 (service not found or mapping rules not found).

3. Formatting is off for this item - there is a wrong backslash:

Application key is invalid or missing (for app\_id/app\_key authentication method)