time/timekeeping: Fix kernel-doc warnings and typos

Fix punctuation, spellos, and kernel-doc warnings:

  timekeeping.h:79: warning: No description found for return value of 'ktime_get_real'
  timekeeping.h:95: warning: No description found for return value of 'ktime_get_boottime'
  timekeeping.h:108: warning: No description found for return value of 'ktime_get_clocktai'
  timekeeping.h:149: warning: Function parameter or struct member 'mono' not described in 'ktime_mono_to_real'
  timekeeping.h:149: warning: No description found for return value of 'ktime_mono_to_real'
  timekeeping.h:255: warning: Function parameter or struct member 'cs_id' not described in 'system_time_snapshot'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240331172652.14086-3-rdunlap@infradead.org