Media: Update some attach/detach strings in the columns view.

* Show a "Media attachment reattached."/"Media attachment detached." message when only one attachment was attached/detached. 
* Use the string placeholder as `number_format_i18n()` returns a string.
* Add translator comments.
* Update help text to refer to the new dropdown menus.
* Add a more generic title to the attach modal.

Fixes #33237.

git-svn-id: https://develop.svn.wordpress.org/trunk@36328 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/template.php

@@ -1483,7 +1483,7 @@ function find_posts_div($found_action = '') {
 ?>
 	<div id="find-posts" class="find-box" style="display: none;">
 		<div id="find-posts-head" class="find-box-head">
-			<?php _e( 'Find Posts or Pages' ); ?>
+			<?php _e( 'Attach to existing content' ); ?>
 			<div id="find-posts-close"></div>
 		</div>
 		<div class="find-box-inside">