mm: add kdoc comments for vma_start/last_pgoff()

Describe what vma_start_pgoff() and vma_last_pgoff() actually provide in
detail.

This is in order that we can differentiate this between functions that will
be added in a subsequent patch which provide a different page offset.

We go to lengths to describe the edge cases that can be run into here.

No functional change intended.

Link: https://lore.kernel.org/20260710-b4-pre-scalable-cow-v2-2-2a5aa403d977@kernel.org
Signed-off-by: Lorenzo Stoakes <ljs@kernel.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: Liam R. Howlett (Oracle) <liam@infradead.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Ackerley Tng <ackerleytng@google.com>
Cc: David Hildenbrand (Arm) <david@kernel.org>
Cc: Kai Huang <kai.huang@intel.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: SJ Park <sj@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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