um: time-travel: clean up kernel-doc warnings

um: time-travel: clean up kernel-doc warnings

Repair all kernel-doc warnings in um_timetravel.h:
- add one enum description
- mark "reserve" as private
- use a leading '@' on current_time

Warning: include/uapi/linux/um_timetravel.h:59 Enum value
 'UM_TIMETRAVEL_SHARED_MAX_FDS' not described in enum
 'um_timetravel_shared_mem_fds'
Warning: include/uapi/linux/um_timetravel.h:245 union member 'reserve'
 not described in 'um_timetravel_schedshm_client'
Warning: include/uapi/linux/um_timetravel.h:288 struct member
 'current_time' not described in 'um_timetravel_schedshm'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patch.msgid.link/20260226221112.1042008-1-rdunlap@infradead.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>