doc: mark assert.CallTracker experimental

Some details might still change and it would be good to get feedback
from users before we mark this as stable.

Signed-off-by: Ruben Bridgewater <ruben@bridgewater.de>

PR-URL: #33124
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

doc/api/assert.md

@@ -150,6 +150,13 @@ try {
 ```
 
 ## Class: `assert.CallTracker`
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental
+
+This feature is currently experimental and behavior might still change.
 
 ### `new assert.CallTracker()`
 <!-- YAML