rewrite attach_dir_desc

Rewrite as per https://github.com/SimpleMachines/SMF2.1/issues/288
<br/> I agree to the DCO 1.1 at https://github.com/SimpleMachines/SMF2.1/commit/b486a1e52a5f4f1b9ffae37defc6cd864926267d#DCO.txt

Themes/default/languages/Admin.english.php

@@ -385,7 +385,7 @@ $txt['attach_dir_empty'] = 'Empty';
 $txt['attach_dir_ok'] = 'OK';
 $txt['attach_dir_basedir'] = 'Base directory';
 
-$txt['attach_dir_desc'] = 'Create new directories or change the current directory below. Directories can be renamed as long as they do not contain a sub-directory. If the new directory is to be in the created within the forum directory structure. Just the directory name can be used. To remove a directory, blank the path input field. Directories can not be deleted if they contain either files or sub-directories (shown in brackets next to the file count).';
+$txt['attach_dir_desc'] = 'Create new directories or change the current directory below. <br />To create a new directory within the forum directory structure, use just the directory name. <br />To remove a directory, blank the path input field. Only empty directories can be removed. To see if a directory is empty, check for files or sub-directories in brackets next to the file count. <br /> To rename a directory, simply change its name in the input field. Only directories without sub-directories may be renamed.'Directories can be renamed as long as they do not contain a sub-directory. If the new directory is to be in the created within the forum directory structure.';
 $txt['attach_dir_base_desc'] = 'You may use below to change the current base directory or create a new one. New base directories are also added to the Attachment Directory list. You may also designate an existing directory to be a base directory.';
 $txt['attach_dir_save_problem'] = 'Oops, there seems to be a problem.';
 $txt['attachments_no_create'] = 'Unable to create a new attachment directory. Please do so using a FTP client or your site file manager.';