Add an example to add more pci devices in nova.conf

The help for the [pci]aliases config option was confusing in that it
wasn't clear how to specify multiple aliases. The default value being []
made it seem like you ought to be able to specify a list as:

 alias = [{...}, {...}, ...]

This won't actually work; you instead have to say:

 alias = {...}
 alias = {...}
 ...

So this patch adds a multi-alias example to the help string and
explicitly calls out the fact that you can't use list values.

See the referenced bug for more details.
Change-Id: I71d4c440340db31d22a73a8b3e3abc51771aa7bc
Closes-Bug: #1786870

nova/conf/pci.py

@@ -33,7 +33,7 @@ needing to repeat all the PCI property requirements.
 
 Possible Values:
 
-* A list of JSON values which describe the aliases. For example::
+* A dictionary of JSON values which describe the aliases. For example::
 
     alias = {
       "name": "QuickAssist",
@@ -62,6 +62,24 @@ Possible Values:
   ``numa_policy``
     Required NUMA affinity of device. Valid values are: ``legacy``,
     ``preferred`` and ``required``.
+
+* Supports multiple aliases by repeating the option (not by specifying
+  a list value)::
+
+    alias = {
+      "name": "QuickAssist-1",
+      "product_id": "0443",
+      "vendor_id": "8086",
+      "device_type": "type-PCI",
+      "numa_policy": "required"
+    }
+    alias = {
+      "name": "QuickAssist-2",
+      "product_id": "0444",
+      "vendor_id": "8086",
+      "device_type": "type-PCI",
+      "numa_policy": "required"
+    }
 """),
     cfg.MultiStrOpt('passthrough_whitelist',
         default=[],