Add notes on X selections (AKA clipboards)

src/libexec/saturnin-www

@@ -7,6 +7,17 @@ ffoo import pretty
 
 ffoo import saturnin_www
 
+#
+# On clipboards:
+#
+#   *  primary is for selection/middle mouse click (X power users)
+#   *  clipboard is for Win/Mac-like explicit Copy/Cut/Paste
+#   *  nobody uses secondary
+#
+# see http://standards.freedesktop.org/clipboards-spec/clipboards-latest.txt
+# for more details
+#
+
 clipln() {
     #
     # Print primary clipboard and \n