Unify documentation on 'save-some-buffers'

* doc/emacs/files.texi (Save Commands): Include some more keys
that can be used in 'save-some-buffers'.

* lisp/files.el (save-some-buffers): Include the same keys as
above, and document all missing ones.

doc/emacs/files.texi

@@ -400,11 +400,14 @@ possible responses are analogous to those of @code{query-replace}:
 
 @table @kbd
 @item y
+@item @key{SPC}
 Save this buffer and ask about the rest of the buffers.
 @item n
+@item @key{DEL}
 Don't save this buffer, but ask about the rest of the buffers.
 @item !
 Save this buffer and all the rest with no more questions.
+@item q
 @c following generates acceptable underfull hbox
 @item @key{RET}
 Terminate @code{save-some-buffers} without any more saving.

lisp/files.el

@@ -5209,9 +5209,14 @@ about certain files that you'd usually rather not save."
 
 (defun save-some-buffers (&optional arg pred)
   "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'.
+You can answer `y' or SPC to save, `n' or DEL not to save, `C-r'
+to look at the buffer in question with `view-buffer' before
+deciding, `d' to view the differences using
+`diff-buffer-with-file', `!' to save the buffer and all remaining
+buffers without any further querying, `.' to save only the
+current buffer and skip the remaining ones and `q' or RET to exit
+the function without saving any more buffers.  `C-h' displays a
+help message describing these options.
 
 This command first saves any buffers where `buffer-save-without-query' is
 non-nil, without asking.