Complete doc.

Author: cjgillot

library/core/src/intrinsics/mod.rs

@@ -2798,7 +2798,11 @@ pub const fn align_of<T>() -> usize;
 /// Therefore, implementations must not require the user to uphold
 /// any safety invariants.
 ///
+/// This intrinsic can only be evaluated at compile-time, and should only appear in
+/// constants or inline const blocks.
+///
 /// The stabilized version of this intrinsic is [`core::mem::offset_of`].
+/// This intrinsic is also a lang item so `offset_of!` can desugar to calls to it.
 #[rustc_nounwind]
 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_const_unstable(feature = "core_intrinsics", issue = "none")]