Spec: Allow updating userBiddingSignals (#907)

spec.bs

@@ -3631,6 +3631,15 @@ The <dfn for=Navigator method>updateAdInterestGroups()</dfn> method steps are:
           set |ig|'s [=interest group/trusted bidding signals keys=] to |value|.
         1. Otherwise, jump to the step labeled <i><a href=#abort-update>Abort update</a></i>.
 
+        <dt>"`userBiddingSignals`"
+        <dd>
+        1. Set |ig|'s [=interest group/user bidding signals=] to the result of [=serialize an Infra
+          value to JSON bytes=] given |value|.
+        1. Otherwise, jump to the step labeled <i><a href=#abort-update>Abort update</a></i>.
+
+        Issue:  Serializing an Infra value to JSON bytes expects to be called within a valid ES realm. See
+        <a href="https://github.com/whatwg/infra/issues/625">infra/625</a>
+
         <dt>"`ads`"
         <dt>"`adComponents`"
         <dd>