Documentation improvements.

Document that every model must have at least one input and one output.

Document graph constraints.

Elaborate a bit on "execution order".

Indicate how operands of different lifetimes are defined (or not).

Fix ANEURALNETWORKS_L2_NORMALIZATION documentation for output shape.

ANEURALNETWORKS_SPACE_TO_DEPTH has inconsistent names for dimensions.

ANEURALNETWORKS_PAD documentation doesn't clearly describe rank or
dimensions of output tensor.

Bug: 66478689
Bug: 67601776
Bug: 69631404
Bug: 79482779
Bug: 79539157
Bug: 79539928

Test: mma at tree root

Change-Id: I6227bea2a486d4b45022d590915e10a124d323d4
Merged-In: I6227bea2a486d4b45022d590915e10a124d323d4