document that the property 'target' for provider 'yumrepo' does not work

Change documentation about the 'target' property for the 'yumrepo' provider

The 'target' property of the yumrepo provider does not work. To make it work would take a bit of work, which we want to do, but it would be best to wait until we've split the provider out of puppet. This ticket is to just change the documentation for 'target' for now and later we'll want to address PUP-2782 to make 'target' work correctly.

"The target parameter will be enabled in a future release and should not be used."