doc: adds missing comment in assert examples
Adds the corresponding missing comment:
// Creates call tracker.
That is present on some but not all examples of the Class: assert.CallTracker
documentation.
I'm leaving out the examples on new assert.CallTracker()
because it's already being described there.
I know it's a small change and it's literally going to be removed soon but still, the comment is not present on all examples as of now.