Document current behavior and don't guarantee 16-bit wide isize/usize

Author: gnzlbg

reference/src/layout/scalars.md

@@ -35,8 +35,14 @@ of `char` is _implementation-defined_.
 
 The `isize` and `usize` types are pointer-sized signed and unsigned integers.
 They have the same layout as the [pointer types] for which the pointee is
-`Sized`, and are layout compatible with C's `uintptr_t` and `intptr_t` types,
-and are therefore at least 16-bit wide.
+`Sized`, and are layout compatible with C's `uintptr_t` and `intptr_t` types.
+
+> **Note**: on all platforms that we currently support, `usize`/`isize` are at
+> least 16-bit wide. C99
+> [7.18.2.4](https://port70.net/~nsz/c/c99/n1256.html#7.18.2.4) requires
+> `uintptr_t` and `intptr_t` to be at least 16-bit wide. `libcore` relies on
+> this to unconditionally provide impls of `From<i16>`/`From<u16>` for
+> `isize`/`usize`.
 
 > **Note**: Rust's `usize` and C's `unsigned` types are **not** equivalent. C's
 > `unsigned` is at least as large as a short, allowed to have padding bits, etc.
@@ -60,23 +66,6 @@ and are therefore at least 16-bit wide.
 > These limits have not gone through the RFC process and are not guaranteed to
 > hold.
 
-<details><summary><b>Rationale</b></summary> 
-
-C99 [7.18.2.4](https://port70.net/~nsz/c/c99/n1256.html#7.18.2.4) "Limits of
-integer types capable of holding object pointers" provides the following minimum
-range of values that the pointer-holding integer types must support:
-
-* `intptr_t`: `[INTPTR_MIN, INTPTR_MAX] = [-(2^15 - 1), 2^15 - 1]` 
-* `uintptr_t`: `[0, UINTPTR_MAX] = [0, 2^16 - 1]`
-
-That is, `isize`/`usize` are at least 16-bit wide. `libcore` relies on
-this guarantee and unconditionally provides:
-
-* `impl From<i16> for isize`,
-* `impl From<u16> for usize`.
-
-</details>
-
 [pointer types]: ./pointers.md
 
 ## Fixed-width integer types