Documentation: Document user_events ioctl code

The user events trace subsystem uses the 0x2A/'*' code for ioctls. These
are published via the uapi/linux/user_events.h header file.

Add a line indicating user events as the owner of the 0x2A/'*' code and
reserve the first 32 sequence numbers.

Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240715181413.1157-1-beaub@linux.microsoft.com