Document as-precedence.rs and downgrade to check-pass

Author: jieyouxu

tests/ui/as-precedence.rs

@@ -1,4 +1,12 @@
-//@ run-pass
+//! Parser precedence test to help with [RFC 87 "Trait Bounds with Plus"][rfc-87], to check the
+//! precedence of the `as` operator in relation to some arithmetic bin-ops and parentheses.
+//!
+//! Editor's note: this test seems quite incomplete compared to what's possible nowadays. Maybe
+//! there's another set of tests whose coverage overshadows this test?
+//!
+//! [rfc-87]: https://rust-lang.github.io/rfcs/0087-trait-bounds-with-plus.html
+
+//@ check-pass
 
 #[allow(unused_parens)]
 fn main() {