👌 add details on a mutationObserver test

Author: BenoitZugmeyer

packages/rum-recorder/src/domain/rrweb/mutationObserver.spec.ts

@@ -145,7 +145,12 @@ describe('startMutationCollection', () => {
       })
     })
 
-    describe('does not emit mutations on freshly serialized nodes and their descendants', () => {
+    describe('does not emit mutations on freshly re-serialized nodes and their descendants', () => {
+      // Note about those tests: any mutation with a not-yet-serialized 'target' will be trivially
+      // ignored. We want to focus on mutations with a 'target' that have already been serialized
+      // (during the document snapshot for example), and re-serialized (by being added in the
+      // document) during the processed mutation batched.
+
       it('attribute mutations', () => {
         const element = document.createElement('div')
         sandbox.appendChild(element)