Fix `tf.io.decode_raw` bugs and update documentation.

Fixes cases where specifying `fixed_length` resulted in data loss and even segfault and corruption of the Python interpreter. The fix is subtle but needed due to pointer arithmetic rules.

Makes sure that `fixed_length` does not change the output when present but not needed.

Eliminates needless copy and cast in the main codepath.

PiperOrigin-RevId: 371322725
Change-Id: I514ef67a2961c86422f69d05122d31615e87896c