fscrypt: improve a few comments

Improve a few comments.  These were extracted from the patch
"fscrypt: add support for hardware-wrapped keys"
(https://lore.kernel.org/r/20211021181608.54127-4-ebiggers@kernel.org).

Link: https://lore.kernel.org/r/20211026021042.6581-1-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>