Reserve some function keys for users.

lispref/tips.texi

@@ -126,6 +126,10 @@ character are allocated for minor modes.  Using them in a major mode is
 not absolutely prohibited, but if you do that, the major mode binding
 may be shadowed from time to time by minor modes.
 
+@item
+Function keys @key{F5} through @key{F9} without modifier keys are
+reserved for users to define.
+
 @item
 You should not bind @kbd{C-h} following any prefix character (including
 @kbd{C-c}).  If you don't bind @kbd{C-h}, it is automatically available