conf: Call out where pci.alias should be set

We could probably do with a more concrete way to specify what services
use a given configuration value, but this is a start.

Change-Id: I23051a48754f2d43867ae9efc28691f766e6c6f3

nova/conf/pci.py

@@ -31,6 +31,9 @@
 This allows users to specify the alias in the extra specs for a flavor, without
 needing to repeat all the PCI property requirements.
 
+This should be configured for the ``nova-api`` service and, assuming you wish
+to use move operations, for each ``nova-compute`` service.
+
 Possible Values:
 
 * A dictionary of JSON values which describe the aliases. For example::