Auto-save all buffers that need it.
This is all buffers that have auto-saving enabled
and are changed since last auto-saved.
Auto-saving writes the buffer into a file
so that your editing is not lost if the system crashes.
This file is not the file you visited; that changes only when you save.
Normally we run the normal hook `auto-save-hook' before saving.
A non-nil NO-MESSAGE argument means do not print any message if successful.
A non-nil CURRENT-ONLY argument means save only current buffer.
(fn &optional NO-MESSAGE CURRENT-ONLY)