Document that old proxy update endpoint creates apibackend

Consider the following call:

curl -vk -X PATCH "https://3scale-admin.<wildcard_domain>/admin/api/services/<id>/proxy.json" -d 'access_token=<access_token>&api_backend=https://echo-api.3scale.net:443/'

Result (in the APIAP scenario)

• the default backend in the installation is updated with the api_backend value and associates it to the proxy
• other backends associated with the Product are removed from the Product and replaced with the default backend in the installation.

This is undocumented behavior.

Expected behaviour

Either of the following 2 scenarios:

1. This is not the desired behaviour and this should not update the api backends as shown in the example.
2. This is the desired behaviour and just needs to be documented.

Please read all the comments below before starting.

Document this better in the 3scale apidocs.