How do I display the search box?

Just use any default WP search box or widget, Relevanssi doesn’t have a separate search box. If you want a search form on a page, you can use this:

add_shortcode( 'searchform', 'rlv_searchform' );
function rlv_searchform() {
    return get_search_form( false );
}

Add this code to your theme functions.php, then you can use [searchform] to add a search form anywhere where shortcodes can be used.

No, Relevanssi can’t see inside documents.