Update src/raw_field.rs

Co-authored-by: Ralf Jung <post@ralfj.de>

src/raw_field.rs

@@ -172,8 +172,8 @@ macro_rules! raw_field_tuple {
 /// This macro is the same as `raw_field`, except for a different Deref-coercion check that
 /// supports unions.
 /// Due to macro_rules limitations, this check will accept structs with a single field as well as unions.
-/// Using this macro for a single-field struct will still work, but might lead to a future
-/// compatibility problem if the struct gains more fields.
+/// This is not a stable guarantee, and future versions of this crate might fail
+/// on any use of this macro with a struct, without a semver bump.
 #[macro_export(local_inner_macros)]
 macro_rules! raw_field_union {
     ($base:expr, $parent:path, $field:tt) => {{