NNAPI: Clarify behaviour of REDUCE_* ops

Add a clarification on the behaviour of REDUCE_* ops when all dimensions
of an input tensor are reduced.

Bug: 155508675
Test: VtsHalNeuralnetworksV1_3TargetTest
Change-Id: I3a67225c3af80af40a11362ca5d0f55350cbfa05
3 files changed