explain that the unaligned_field_offset intrinsic is const-only

Author: BennoLossin

library/core/src/intrinsics/mod.rs

@@ -3315,7 +3315,14 @@ pub unsafe fn va_arg<T: VaArgSafe>(ap: &mut VaListImpl<'_>) -> T;
 #[rustc_nounwind]
 pub unsafe fn va_end(ap: &mut VaListImpl<'_>);
 
-/// `offset_of!` for `UnalignedField` trait.
+/// `offset_of!` for field representing types.
+///
+/// Returns the offset of the field represented by `F`.
+///
+/// Expects the generic `F` to be a field representing type expressed using
+/// [`field_of!`](core::field::field_of).
+///
+/// This is a `const`-only intrinsic that doesn't have any codegen.
 #[rustc_intrinsic]
 #[lang = "unaligned_field_offset"]
 #[unstable(feature = "field_projections", issue = "145383")]