fscrypt: document inline encryption support

Update the fscrypt documentation file for inline encryption support.

Signed-off-by: Satya Tangirala <satyat@google.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Link: https://lore.kernel.org/r/20200724184501.1651378-7-satyat@google.com


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