Spec the finally() operator

spec.bs

@@ -1545,7 +1545,33 @@ For now, see
 <div algorithm>
   The <dfn for=Observable method><code>finally(|callback|)</code></dfn> method steps are:
 
-    1. <span class=XXX>TODO: Spec this and use |callback|.</span>
+    1. Let |sourceObservable| be [=this=].
+
+    1. Let |observable| be a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
+       algorithm that takes a {{Subscriber}} |subscriber| and does the following:
+
+       1. Run |subscriber|'s {{Subscriber/addTeardown()}} method with |callback|.
+
+       1. Let |sourceObserver| be a new [=internal observer=], initialized as follows:
+
+          : [=internal observer/next steps=]
+          :: Run |subscriber|'s {{Subscriber/next()}} method, given the passed in <var
+             ignore>value</var>.
+
+          : [=internal observer/error steps=]
+          :: 1. Run |subscriber|'s {{Subscriber/error()}} method, given the passed in <var
+                ignore>error</var>.
+
+          : [=internal observer/complete steps=]
+          :: 1. Run |subscriber|'s {{Subscriber/complete()}} method.
+
+       1. Let |options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is
+          |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=].
+
+       1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |sourceObservable|
+          given |sourceObserver| and |options|.
+
+    1. Return |observable|.
 </div>