for blogger importer, add docs and fix how-to-use message in jekyll import blogger

Usage:

  jekyll import blogger [options]

Options:
            --source NAME  The XML file (blog-MM-DD-YYYY.xml) path to import
            --no-blogger-info  Leave blogger-URL info (id and old URL.) as YAML data (default: false)
            --replace-internal-link  Whether to replace internal links with the post_url liquid codes (default: false)
        -h, --help         Show this message
        -v, --version      Print the name and version
        -t, --trace        Show the full backtrace when an error occurs