Create PA_Feature_Detecting.md (#910)

For previously documented, explained and spec'd features, provide developers with help for how to feature detect these features.

PA_Feature_Detecting.md

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+# Feature detecting Protected Audience
+
+As Chrome ships and experiments with various Protected Audience APIs, it can be useful for web developers to detect the presence of individual features
+in users’ browsers.  This document seeks to list feature detection mechanisms for each shipped or experimented feature.
+
+## Protected Audience
+[Intent to Ship](https://groups.google.com/a/chromium.org/g/blink-dev/c/igFixT5n7Bs/m/ZNrDcQ2dDQAJ)
+```
+let pa = typeof navigator.joinAdInterestGroup !== 'undefined';
+```
+## directFromSellerSignals via headers
+[Intent to Ship](https://groups.google.com/a/chromium.org/g/blink-dev/c/JpWOdoPi5Wo/m/YyTaUzkxAgAJ)
+```
+let dfss = false;
+navigator.runAdAuction({get directFromSellerSignalsHeaderAdSlot(){dfss = true;}}).catch((e)=>{});
+```
+## Negative targeting
+[Intent to Ship](https://groups.google.com/a/chromium.org/g/blink-dev/c/xzrWfs-BwFk/m/a90JCji_AAAJ)
+```
+let nt = typeof navigator.createAuctionNonce !== 'undefined';
+```
+## clearOriginJoinedAdInterestGroups()
+[Intent to Ship](https://groups.google.com/a/chromium.org/g/blink-dev/c/IfmYsMCUoHc/m/yCddTUfgBgAJ)
+```
+let nt = typeof navigator.clearOriginJoinedAdInterestGroups !== 'undefined';
+```
+## Interest group limit changes
+[Intent to Ship](https://groups.google.com/a/chromium.org/g/blink-dev/c/IfmYsMCUoHc/m/yCddTUfgBgAJ)
+
+These limits are intended to be guardrails against exceptional behavior, and not meant to be reached under normal conditions, so there should not be a 
+need to find out what the guardrails are.  In [the best practices we previously published](https://developer.chrome.com/docs/privacy-sandbox/protected-audience-api/latency/#fewer-interest-groups-bidding),
+we encourage buyers to use fewer interest groups.
+## kAnonStatus
+[Intent to Ship](https://groups.google.com/a/chromium.org/g/blink-dev/c/IfmYsMCUoHc/m/yCddTUfgBgAJ)
+```
+reportWin(auctionSignals, perBuyerSignals, sellerSignals, browserSignals, directFromSellerSignals) {
+  ...
+  let ka = typeof browserSignals.kAnonStatus !== 'undefined';
+}
+```
+## Bidding & Auction Services
+[Intent to Experiment](https://groups.google.com/a/chromium.org/g/blink-dev/c/2bwMHd3Yz7I/m/BwMKwPP6GQAJ) (note that this is an
+[Origin Trial](https://developer.chrome.com/en/docs/web-platform/origin-trials/) and as such
+requires [Origin Trial tokens](https://developer.chrome.com/en/docs/web-platform/origin-trials/#iframe))
+```
+let ba = typeof navigator.getInterestGroupAdAuctionData !== 'undefined';
+```
+## Recency in generateBid()
+[Intent to Ship](https://groups.google.com/a/chromium.org/g/blink-dev/c/-bQKNLit6nw/m/vPe0uSXtAAAJ)
+```
+generateBid(interestGroup, auctionSignals, perBuyerSignals,
+    trustedBiddingSignals, browserSignals, directFromSellerSignals) {
+  ...
+  let rg = typeof browserSignals.recency !== 'undefined';
+}
+```
+## Rounding bids and scores
+[Intent to Ship](https://groups.google.com/a/chromium.org/g/blink-dev/c/-bQKNLit6nw/m/vPe0uSXtAAAJ)
+
+This is a privacy protection that does not require different behavior from Protected Audience API callers and hence has no feature detection mechanism.
+## Credentialed automatic beacons
+[Intent to Ship](https://groups.google.com/a/chromium.org/g/blink-dev/c/rMyTWCo-f_I)
+
+Whether automatic beacons will attach credentials cannot be detected from inside Protected Audience ads. From the perspective of the server, the request
+has either credentials mode “omit” (feature disabled) or “include” (feature enabled), which affects how headers are processed in the same way as the rest
+of the web platform.
+## reserved.top_navigation_start/commit
+This cannot be detected from the web platform. Instead, an ad auction can register both a `reserved.top_navigation_commit` and a `reserved.top_navigation`
+beacon, and then the ad frame can set the same automatic beacon data for both event types. The beacon destination server can then compare
+`top_navigation_commit` and `top_navigation` beacons, and filter out duplicate beacons that have the same exact data.