Add a comment explaining the split

Signed-off-by: Jens Reidel <adrian@travitia.xyz>

Author: Gelbpunkt

tests/ui/codegen/duplicated-path-in-error.rs

@@ -6,6 +6,9 @@
 
 // This test ensures that the error of the "not found dylib" doesn't duplicate
 // the path of the dylib.
+//
+// glibc and musl have different dlopen error messages, so the expected error
+// message differs between the two.
 
 fn main() {}