The 'f' ioctls with numbers 19-26 decimal are currently used for fscrypt
(a.k.a. ext4/f2fs/ubifs encryption), and up to 39 decimal is reserved
for future fscrypt use, as per the comment in fs/ext4/ext4.h.  So the
reserved range is 13-27 hex.

Document this in ioctl-number.rst.
Signed-off-by: NEric Biggers <ebiggers@google.com>