mm: hugetlb_vmemmap: move code comments to vmemmap_dedup.rst

All the comments which explains how HVO works are moved to
vmemmap_dedup.rst since

  commit 4917f55b

 ("mm/sparse-vmemmap: improve memory savings for compound devmaps")

except some comments above page_fixed_fake_head().  This commit moves
those comments to vmemmap_dedup.rst and improve vmemmap_dedup.rst as well.

Link: https://lkml.kernel.org/r/20220628092235.91270-8-songmuchun@bytedance.com
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Xiongchun Duan <duanxiongchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>