FROMLIST: docs: ioctl-number: document fscrypt ioctl numbers

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.

Change-Id: Id106c59de21d1585c22729bd5b4339b27c298c9c
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Satya Tangirala <satyat@google.com>
Link: https://patchwork.kernel.org/patch/11182409/