sh: Document PTEL 31:29 use on PTEA-wielding parts.

 * - Bits 10 and 11 are low bits of the PPN that are reserved on >= 4K pages.

 *   Bit 10 is used for _PAGE_ACCESSED, bit 11 remains unused.

 *

 * - Bits 31, 30, and 29 remain unused by everyone and can be used for future

 *   software flags, although care must be taken to update _PAGE_CLEAR_FLAGS.

 * - On 29 bit platforms, bits 31 to 29 are used for the space attributes

 *   and timing control which (together with bit 0) are moved into the

 *   old-style PTEA on the parts that support it.

 *

 * XXX: Leave the _PAGE_FILE and _PAGE_WT overhaul for a rainy day.

 *