kexec: update manpage with explicit mention of clean kexec
    
    Resolves: RHEL-14000
    Upstream: https://github.com/horms/kexec-tools
    Conflict: None
    
    commit bd0200c47c45dd420244b39ddabcecdab1fb9a8e
    Author: Hari Bathini <hbathini@linux.ibm.com>
    Date:   Wed Sep 20 17:29:27 2023 +0530
    
        kexec: update manpage with explicit mention of clean kexec
    
        While the manpage does mention about kexec boot with a clean shutdown,
        it is not explicit about it. Make it explicit.
    
        Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
        Signed-off-by: Simon Horman <horms@kernel.org>
    
    Signed-off-by: Lichen Liu <lichliu@redhat.com>