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 <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: KAGA-KOKO