move libstd concerns out of the rustc note

Author: RalfJung

src/behavior-considered-undefined.md

@@ -53,9 +53,10 @@ code.
       (i.e., it must not be read from uninitialized memory).
   * Non-UTF-8 byte sequences in a `str`.
   * Invalid values for a type with a custom definition of invalid values.
+    In the standard library, this affects [`NonNull<T>`] and [`NonZero*`].
 
-    > **Note**: For `rustc`, those types are [`NonNull<T>`] and [`NonZero*`].
-    > Requesting custom invalid values requires the unstable `rustc_layout_scalar_valid_range_*` attributes.
+    > **Note**: For `rustc`, requesting custom invalid values requires the
+    > unstable `rustc_layout_scalar_valid_range_*` attributes.
 
 A reference/pointer is "dangling" if it is null or not all of the bytes it
 points to are part of the same allocation (so in particular they all have to be