Allows num_parallel_calls to be set to AUTOTUNE and adds more comments.
Allows num_parallel_calls to be set to AUTOTUNE and adds more comments.

Partially addresses https://github.com/deepmind/acme/issues/186 and the proposal in https://github.com/deepmind/acme/pull/193

PiperOrigin-RevId: 430058577
Change-Id: I4221bd6dcc5388debb70be8505e7b01a942f87cb

deepmind/acme,deepmind/acme