StableHLO Spec Improvements and Clarifications

[GleasonK]

Request description

From OpenXLA Discord

• It seems to be, that the stablehlo specs don't define array<...> (https://openxla.org/stablehlo/spec). Am I missing something here? From the examples it looks like it is a tensor constant, but I can't find its definition in the specification.
• Also I think it's a bit underspecified how to specify the function arguments for ops like if and case, i.e. where it's apparently required/allowed (?) to discard the Unused part and instead of actually providing a Func, just provide a "{" {Op} "}"
• It was also not super clear to me that the functions that are passed to "stablhlo.case" can capture variables from their enclosing environment, at least I did not find the docs for it.