time: add kernel-doc in time.c

Add kernel-doc for all APIs that do not already have it.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: John Stultz <jstultz@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230704052405.5089-3-rdunlap@infradead.org