mm/doc: add usage description for migrate_vma_*()

The migrate_vma_setup(), migrate_vma_pages(), and migrate_vma_finalize()
API usage by device drivers is not well documented.
Add a description for how device drivers are expected to use it.

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Link: https://lore.kernel.org/r/20200909212956.20104-1-rcampbell@nvidia.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>