time/timecounter: Fix inline documentation

Fix kernel-doc warnings, text punctuation, and a kernel-doc marker
(change '%' to '&' to indicate a struct):

  timecounter.h:72: warning: No description found for return value of 'cyclecounter_cyc2ns'
  timecounter.h:85: warning: Function parameter or member 'tc' not described in 'timecounter_adjtime'
  timecounter.h:111: warning: No description found for return value of 'timecounter_read'
  timecounter.h:128: warning: No description found for return value of 'timecounter_cyc2time'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240331172652.14086-2-rdunlap@infradead.org