Documentation: core-api/cpuhotplug: Rewrite the API section

Dave stumbled over the incomplete and confusing documentation of the CPU
hotplug API.

Rewrite it, add the missing function documentations and correct the
existing ones.

Reported-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210909123212.489059409@linutronix.de