library: core: document layout guarantee of TypeId

Document the layout guarantee of `TypeId` not to exceed 16 bytes.

This is useful for FFI use-cases where a `TypeId` may be stored within a
foreign data structure or buffer.

An example for such a use-case can be found in [1].

Link: https://git.kernel.org/pub/scm/linux/kernel/git/driver-core/driver-core.git/commit/?id=6f61a2637abe4f89877da3280775565baedb60e0 [1]
Suggested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: Danilo Krummrich

library/core/src/any.rs

@@ -611,6 +611,10 @@ impl dyn Any + Send + Sync {
 /// noting that the hashes and ordering will vary between Rust releases. Beware
 /// of relying on them inside of your code!
 ///
+/// # Layout
+///
+/// The size of `TypeId` is guaranteed not to exceed 16 bytes.
+///
 /// # Danger of Improper Variance
 ///
 /// You might think that subtyping is impossible between two static types,