* mu4e: update the mail/indexing update process:

- M-x mu4e-update-mail-and-index (C-S-U in main/headers/view/compose, with
  prefix arg, run in background
  - M-x mu4e-update-index to only update the index
  - document / add to FAQ

mu4e/mu4e-headers.el

@@ -443,6 +443,8 @@ after the end of the search results."
   (setq mu4e-headers-mode-map
     (let ((map (make-sparse-keymap)))
 
+      (define-key map  (kbd "C-S-u") 'mu4e-update-mail-and-index)
+      
       (define-key map "s" 'mu4e-headers-search)
       (define-key map "S" 'mu4e-headers-search-edit)