pkru.3: Remove a qualifier

Now that i386 kernels are deprecated, we don't really need to mention
this limitation. It's also a bit dated since PKRU is supported with
5-level paging as well.

Reviewed by: alc, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D56415

(cherry picked from commit fe6bf738aaeb3d5fd74aabfcbf01eba827df6594)