add man documentation for kdumpctl get-default-crashkernel
    
    Resolves: bz2073676
    Upstream: Fedora
    Conflict: None
    
    commit a1c63fa6449740f771349f931748b0ccc8a67ec7
    Author: Coiby Xu <coxu@redhat.com>
    Date:   Tue Mar 1 17:35:39 2022 +0800
    
        add man documentation for kdumpctl get-default-crashkernel
    
        A few typos and grammar issues are fixed as well.
    
        Philipp Rudo <prudo@redhat.com>
    
        Signed-off-by: Coiby Xu <coxu@redhat.com>
    
    Signed-off-by: Coiby Xu <coxu@redhat.com>