How To Create a Metabox of HTML Content with Instructions For Editors When Editing a Post or Page?

Don’t know any plugin for that. But it can be solved creating a custom plugin with the Settings API and a Custom Meta Box.

Adding a RichText Editor in the Settings Page

^{/wp-admin/options-general.php}

<?php
/* Plugin Name: Custom Editor in Settings Page */

add_action( 'admin_init', 'register_settings_wpse_57647' );

# Register settings
function register_settings_wpse_57647() 
{
    register_setting( 
        'general', 
        'html_guidelines_message',
        'esc_html'
    );
    add_settings_section( 
        'site-guide', 
        'Publishing Guidelines', 
        '__return_false', 
        'general' 
    );
    add_settings_field( 
        'html_guidelines_message', 
        'Enter custom message', 
        'print_text_editor_wpse_57647', 
        'general', 
        'site-guide' 
    );
}    

# Print settings field content
function print_text_editor_wpse_57647() 
{
    $the_guides = html_entity_decode( get_option( 'html_guidelines_message' ) );
    echo wp_editor( 
        $the_guides, 
        'sitepublishingguidelines', 
        array( 'textarea_name' => 'html_guidelines_message' ) 
    );
}

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Adding a Meta Box to Posts/Pages

This will print the option saved in the Settings page.

add_action( 'add_meta_boxes', 'add_custom_box_wpse_57647' );

# Add Meta Boxes
function add_custom_box_wpse_57647() 
{
    foreach( array( 'post', 'page' ) as $cpt )
        add_meta_box( 
            'mb_wpse_57647',
            __( 'Additional Guidelines', 'this_plugin_or_theme_textdomain' ),
            'inner_custom_box_wpse_57647',
            $cpt,
            'side',
            'high'
        );
}

# Prints the box content
function inner_custom_box_wpse_57647( $post ) 
{
    global $current_user;
    get_currentuserinfo();
    $the_guides = html_entity_decode( get_option ( 'html_guidelines_message' ) );
    echo '<h2>Howdy, ' . $current_user->display_name . '</h2>';
    echo "<div>$the_guides</div><br style="clear:both" />";
}

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