Clarify trusted bidding signals request coalescing (#727)

* Update FLEDGE.md

Looking for clarity disguised as commit, not clear if this is done automatically or if the IG owner needs to take some action (i.e. include interestGroupNames as a key in the trusted keys or some such.

* add note on what Chrome does

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Co-authored-by: Paul Jensen <[email protected]>

FLEDGE.md

@@ -704,7 +704,7 @@ Buyers may want to make on-device decisions that take into account real-time dat
 
 The base URL `https://www.kv-server.example/getvalues` comes from the interest group's `trustedBiddingSignalsURL`, the hostname of the top-level webpage where the ad will appear `publisher.com` is provided by the browser, `experimentGroupId` comes from `perBuyerExperimentGroupIds` if provided, `keys` is a list of `trustedBiddingSignalsKeys` strings, and `interestGroupNames` is a list of the names of the interest groups that data is being fetched for.  `trustedBiddingSignalsSlotSizeMode` is one of `none` (which is the default), `slot-size`, and `all-slots-requested-sizes`.  In the second case, `&slotSize=<width>,<height>` is appended to the URL, where width and height are the normalized width and height from the `requestedSize` of the (component) auction's AuctionConfig.  "Normalized" means units are always appended, and trailing zeros are removed, so {width: "62.50sw", height: "10.0"} becomes "62.5sw,10px".  In the `all-slots-requested-sizes` case, `&allSlotsRequestedSizes=<width1>,<height1>,<width2>,<height2>,...` is appended, where all sizes are taken from the (component) auction's `allSlotsRequestedSizes` value.  If the corresponding value is not present in the auction configuration, no value is appended.
 
-The requests may be coalesced (for efficiency) across a certain number of interest groups that share a `trustedBiddingSignalsURL` and `trustedBiddingSignalsSlotSizeMode` (which means they share an owner). The number of interest groups coalesced into a single request is limited by their `maxTrustedBiddingSignalsURLLength` fields. For instance, if an interest group has a `maxTrustedBiddingSignalsURLLength` of 1000, it means that the length of the trusted bidding signals request URL for this interest group cannot exceed 1000 characters. This prevents requests from being dropped by trusted signals servers due to oversized URLs. If an interest group wants an infinite length for the request URL, it can specify 0 for the `maxTrustedBiddingSignalsURLLength`.
+The requests may automatically be coalesced (for efficiency) across a certain number of interest groups that share a `trustedBiddingSignalsURL` and `trustedBiddingSignalsSlotSizeMode` (which means they share an owner). [Chrome coalesces requests for all interest groups that share an executor.](PA_implementation_overview.md#trusted-signals-fetches--reprioritization) The number of interest groups coalesced into a single request is limited by their `maxTrustedBiddingSignalsURLLength` fields. For instance, if an interest group has a `maxTrustedBiddingSignalsURLLength` of 1000, it means that the length of the trusted bidding signals request URL for this interest group cannot exceed 1000 characters. This prevents requests from being dropped by trusted signals servers due to oversized URLs. If an interest group wants an infinite length for the request URL, it can specify 0 for the `maxTrustedBiddingSignalsURLLength`.
 
 The response from the server should be a JSON object of the form: