AN bidder docs: note required pub-placement match (#459)

* AN bidder docs: note required pub-placement match

* Update based on reviewer feedback

dev-docs/bidders/appnexus-ast.md

@@ -19,6 +19,10 @@ adapter include:
 - Will continue to be upgraded as enhancements are made to Prebid.js
   for expanding capabilities
 
+{: .alert.alert-danger :}
+All AppNexus placements included in a single call to `requestBids` must belong to the same publisher object.  If placements from two different publishers are included in the call, the AppNexus bidder may not return any demand for those placements. <br />
+*Note: This requirement does not apply to adapters that are [aliasing]({{site.baseurl}}/dev-docs/publisher-api-reference.html#module_pbjs.aliasBidder) the AppNexus adapter.*
+
 ### bid params
 
 {: .table .table-bordered .table-striped }