[XLA] Reject wrong number or shape of params in HloEvaluator.

Previously, this would die -- if you were lucky -- somewhere in the depths of
HloEvaluator.

Also tighten up the documentation a bit and remove unused overload for
evaluating a single instruction with some operands as kParameters and others as
kConstants.  This wasn't used anywhere and was easy to misuse.

PiperOrigin-RevId: 227065387