timers: Fix kernel-doc format and add Return values

Fix kernel-doc format and warnings:

  timer.h:26: warning: Cannot understand  * @TIMER_DEFERRABLE: A deferrable timer will work normally when the on line 26 - I thought it was a doc line
  timer.h:146: warning: No description found for return value of 'timer_pending'
  timer.h:180: warning: No description found for return value of 'del_timer_sync'
  timer.h:193: warning: No description found for return value of 'del_timer'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240331172652.14086-4-rdunlap@infradead.org