fscrypt: update documentation for direct I/O support

Now that direct I/O is supported on encrypted files in some cases,
document what these cases are.

Link: https://lore.kernel.org/r/20220128233940.79464-6-ebiggers@kernel.org


Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Biggers <ebiggers@google.com>