This command first saves any buffers where `buffer-save-without-query' is
non-nil, without asking.
-Optional argument (the prefix) non-nil means save all with no questions.
+Optional argument ARG (interactively, prefix argument) non-nil means save
+all with no questions.
Optional second argument PRED determines which buffers are considered:
If PRED is nil, all the file-visiting buffers are considered.
If PRED is t, then certain non-file buffers will also be considered.