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: https://github.com/nodejs/node/pull/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>