pub API: added requestBids.auctionId (prebid#1225)

Author: bretg

dev-docs/publisher-api-reference.md

@@ -565,6 +565,7 @@ Request bids. When `adUnits` or `adUnitCodes` are not specified, request bids fo
 | requestObj.timeout | Optional | `Integer` | Timeout for requesting the bids specified in milliseconds |
 | requestObj.bidsBackHandler | Optional | `function` | Callback to execute when all the bid responses are back or the timeout hits. Callback will be passed two parameters, the bids themselves and `timedOut`, which will be true if any bidders timed out. |
 | requestObj.labels | Optional | `Array of strings` | Defines [labels](#labels) that may be matched on ad unit targeting conditions. |
+| requestObj.auctionId | Optional | `String` | Defines an auction ID to be used rather than having the system generate one. This can be useful if there are multiple wrappers on a page and a single auction ID is desired to tie them together in analytics. |
 
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