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.

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

Documentation/ioctl/ioctl-number.rst

@@ -233,6 +233,7 @@ Code  Seq#    Include File                                           Comments
 'f'   00-0F  fs/ext4/ext4.h                                          conflict!
 'f'   00-0F  linux/fs.h                                              conflict!
 'f'   00-0F  fs/ocfs2/ocfs2_fs.h                                     conflict!
+'f'   13-27  linux/fscrypt.h
 'f'   81-8F  linux/fsverity.h
 'g'   00-0F  linux/usb/gadgetfs.h
 'g'   20-2F  linux/usb/g_printer.h