mm,swap: add document about RCU read lock and swapoff interaction

During reviewing a patch to fix the race condition between
free_swap_and_cache() and swapoff() [1], it was found that the document
about how to prevent racing with swapoff isn't clear enough.  Especially
RCU read lock can prevent swapoff from freeing data structures.  So, the
document is added as comments.

[1] https://lore.kernel.org/linux-mm/[email protected]/

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: "Huang, Ying" <[email protected]>
Reviewed-by: Ryan Roberts <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Minchan Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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