highmem: add kernel-doc for memcpy_*_folio()

This was inadvertently skipped when adding the new functions.

Link: https://lkml.kernel.org/r/20240124181217.1761674-1-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>