Add null documentation comment (#3874)

Author: normj

generator/.DevConfigs/a5da6abb-f0f9-4bb2-b618-4a66aa513ff5.json

@@ -0,0 +1,9 @@
+{
+  "core": {
+    "updateMinimum": true,
+    "type": "patch",
+    "changeLogMessages": [
+      "Generator change to add a comment on collection properties about the V4 change for nullability."
+    ]
+  }
+}