qapi: clarify that the default is backend dependent

qapi: clarify that the default is backend dependent

The default value of the @share option of the @MemoryBackendProperties
really depends on the backend type, so let's document the default
values in the same place where we define the option to avoid
dispersing the information.

Cc: David Hildenbrand <david@redhat.com>
Suggested-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20240618100043.144657-2-sgarzare@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>