Add Additional Bids and Negative Targeting to the Explainer.

The design for additional bids and negative targeting was first
discussed at WICG#319.
A short summary of these featues follows.

In online ad auctions for ad space, it’s sometimes useful to prevent showing an ad to certain audiences, a concept known as negative targeting. For example, you might not want to show a new customer advertisement to existing customers. New customer acquisition campaigns most often have this as a critical requirement. We enable this new use case in the Protected Audience API by allowing buyers to provide "additional bids", which are not generated during the auction based on InterestGroups membership like other bids, but are instead provided as part of the seller's signals request. Today, the seller uses this request to run their contextual auction and return a single winning ad that sets a lower bar that remarketing ads must beat to be shown. In this new model, the seller instead forwards some number of additional bids on behalf of the buyer to the auction running in the user's browser. Each additional bid is allowed to identify one or more InterestGroups that can be used for negative targeting. If the user has been joined to any of the identified InterestGroups, the additional bid is dropped. All remaining additional bids participate in the auction alongside bids generated by InterestGroup membership. Special care is taken to ensure that additional bids are only used in the auction for which the buyer intended them, that these bids are not modified in transit, and that the InterestGroups used for negative targeting are only usable by their owner. In this way, we're enabling advertisers to target new groups of users using the existing privacy-preserving concepts of the Protected Audience API.

FLEDGE.md

@@ -896,7 +896,7 @@ To facilitate negative targeting in Protected Audience auctions, each additional
 
 ##### 6.2.1 Negative Interest Groups
 
-Though negative interest groups are joined using the same `joinAdInterestGroup` API as normal interest groups, they remain distinct from one another. Only negative interest groups can provide an `additionalBidKey`, and only normal interest groups can provide `ads`; no interest group may provide both. The `additionalBidKey` field is described in more detail in section [6.2.3 Additional Bid Keys](#623-additional-bid-keys).
+Though negative interest groups are joined using the same `joinAdInterestGroup` API as regular interest groups, they remain distinct from one another. Only negative interest groups can provide an `additionalBidKey`, and only regular interest groups can provide `ads`; no interest group may provide both. The `additionalBidKey` field is described in more detail in section [6.2.3 Additional Bid Keys](#623-additional-bid-keys).
 
 ```
 const myGroup = {