system/runstate: Document qemu_add_vm_change_state_handler_prio* in hdr

system/runstate: Document qemu_add_vm_change_state_handler_prio* in hdr

Generally APIs to the rest of QEMU should be documented in the headers.
Comments on individual functions or internal details are fine to live
in the C files. Make qemu_add_vm_change_state_handler_prio[_full]()
docstrings consistent by moving them from source to header.

Suggested-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20250715171920.89670-1-philmd@linaro.org>