Interactively, this command sets the primary selection. Without
prefix argument, it reads the selection in the minibuffer. With
-prefix argument, it uses the text of the region as the selection value ."
+prefix argument, it uses the text of the region as the selection value.
+
+Note that on MS-Windows, primary and secondary selections set by Emacs
+are not available to other programs."
(interactive (if (not current-prefix-arg)
(list 'PRIMARY (read-string "Set text for pasting: "))
(list 'PRIMARY (buffer-substring (region-beginning) (region-end)))))