Add some internal documentation on the difference between def_function.py and function.py
Add some internal documentation on the difference between def_function.py and function.py

This is a common source of confusion; the public docstring for tf.function is pretty good, but the internal documentation has been quite sparse.

PiperOrigin-RevId: 358837475
Change-Id: I961dfa70ca8f73c7641a376b09015f5a938050f1

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