Documentation improvements.

Fix Model::pools documentation.

Specify that an output cannot overlap with an output/input/constant.

Document circumstances under which a prepared model should be expected to execute correctly.

Bug: 87805381
Bug: 79888238
Bug: 77237715

Test: mma at tree root

Change-Id: Ibf3daacad79f1d7e6127acf3887bd98b7c6bdf4e
Merged-In: Ibf3daacad79f1d7e6127acf3887bd98b7c6bdf4e