tests: explain why two-panic-runtimes.rs ignores target-dependent errors

Author: jieyouxu

tests/ui/panic-runtime/two-panic-runtimes.rs

@@ -1,11 +1,19 @@
 // ignore-tidy-linelength
 //@ build-fail
-//@ dont-require-annotations: ERROR
 //@ dont-check-compiler-stderr
 //@ aux-build:panic-runtime-unwind.rs
 //@ aux-build:panic-runtime-unwind2.rs
 //@ aux-build:panic-runtime-lang-items.rs
 
+// NOTE: there can be additional errors regarding trying to mix this crate if the precompiled target
+// (such as `wasm32-unknown-unknown` currently unconditionally defaulting to panic=abort) panic
+// strategy differs to abort, then involving a potentially-unwinding `panic_runtime_unwind` that
+// uses a different panic strategy. These errors are important but not to the test intention, which
+// is to check that trying to bring two panic runtimes (`panic_runtime_unwind`) and
+// (`panic_runtime_unwind2`) is prohibited. As such, the additional errors are not checked in this
+// test.
+//@ dont-require-annotations: ERROR
+
 #![no_std]
 #![no_main]
 
@@ -16,7 +24,3 @@ extern crate panic_runtime_lang_items;
 fn main() {}
 
 //~? ERROR cannot link together two panic runtimes: panic_runtime_unwind and panic_runtime_unwind2
-// FIXME: These errors are target-dependent, could be served by some "optional error" annotation
-// instead of `dont-require-annotations`.
-//FIXME~? ERROR the linked panic runtime `panic_runtime_unwind2` is not compiled with this crate's panic strategy `abort`
-//FIXME~? ERROR the crate `panic_runtime_unwind` requires panic strategy `unwind` which is incompatible with this crate's strategy of `abort`