CP-24903: Improve description of Network.purposes
CP-24903: Improve description of Network.purposes

Make it more explicit that the server will use the network
differently depending on the value of this field.

Signed-off-by: Thomas Sanders <c547603d2dd860735c4f0c043b1348f5c48c02f8@citrix.com>

srowe/xen-api,srowe/xen-api,srowe/xen-api,srowe/xen-api,srowe/xen-api