Clarify fail!() semantics/behavior

We recently had some confusion over the behavior of the fail() predicate in XLS.

Consider the pseudo DSLX:
```
fn foo(a: u32) -> bool {
let x = bar(a); <-- Will fail!() on a == alpha
let y = baz(a); <-- Will never fail!()
y if a == alpha else x
}
```

A user expected that this code would never fail, whereas our targeted use case was that it would not. Our understanding is that the sort-of matching pseudoSystemVerilog could also fail:
```
module foo
input a;
output o;
X = bar(a); <= asserts on input alpha
Y = baz(a); <= does not assert on alpha
o = Y if a == alpha else X
endmodule
```

I'll double-check that SV's behavior matches our expectations, and if so, I'll update our docs to make that clear, I suppose.
If it does NOT match our expectations, then...ick.