add additional comments explaining why the IMAP special folders are set up as they are

conf/dovecot-mailboxes.conf

@@ -10,9 +10,13 @@ namespace inbox {
   # Automatically create & subscribe some folders.
   # * Create and subscribe the INBOX folder.
   # * Our sieve rule for spam expects that the Spam folder exists.
+  # * Z-Push must be configured with the same settings in conf/zpush/backend_imap.php (#580).
+
+  # MUA notes:
   # * Roundcube will show an error if the user tries to delete a message before the Trash folder exists (#359).
   # * K-9 mail will poll every 90 seconds if a Drafts folder does not exist.
-  # * Apple's OS X Mail app will create 'Sent Messages' if it doesn't see a folder with the \Sent flag. (#571, #573)
+  # * Apple's OS X Mail app will create 'Sent Messages' if it doesn't see a folder with the \Sent flag (#571, #573) and won't be able to archive messages unless 'Archive' exists (#581).
+  # * Thunderbird's default in its UI is 'Archives' (plural) but it will configure new accounts to use whatever we say here (#581).
 
   # auto:
   # 'create' will automatically create this mailbox.

conf/zpush/backend_imap.php

@@ -15,6 +15,7 @@ define('IMAP_AUTOSEEN_ON_DELETE', false);
 define('IMAP_FOLDER_CONFIGURED', true);
 define('IMAP_FOLDER_PREFIX', '');
 define('IMAP_FOLDER_PREFIX_IN_INBOX', false);
+// see our conf/dovecot-mailboxes.conf file for IMAP special flags settings
 define('IMAP_FOLDER_INBOX', 'INBOX');
 define('IMAP_FOLDER_SENT', 'SENT');
 define('IMAP_FOLDER_DRAFT', 'DRAFTS');