mm: clarify the spec for set_ptes()

Patch series "Transparent Contiguous PTEs for User Mappings", v6.

This is a series to opportunistically and transparently use contpte
mappings (set the contiguous bit in ptes) for user memory when those
mappings meet the requirements.  The change benefits arm64, but there is
some (very) minor refactoring for x86 to enable its integration with
core-mm.

It is part of a wider effort to improve performance by allocating and
mapping variable-sized blocks of memory (folios).  One aim is for the 4K
kernel to approach the performance of the 16K kernel, but without breaking
compatibility and without the associated increase in memory.  Another aim
is to benefit the 16K and 64K kernels by enabling 2M THP, since this is
the contpte size for those kernels.  We have good performance data that
demonstrates both aims are being met (see below).

Of course this is only one half of the change.  We require the mapped
physical memory to be the correct size and alignment for this to actual...