Save some modified file-visiting buffers. Asks user about each one.
You can answer `y' to save, `n' not to save, `C-r' to look at the
buffer in question with `view-buffer' before deciding or `d' to
view the differences using `diff-buffer-with-file'.
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 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.
If PRED is a zero-argument function, it indicates for each buffer whether
to consider it or not when called with that buffer current.
See `save-some-buffers-action-alist' if you want to change the additional actions you can take on files.