Explain style of "done" messages.

lispref/tips.texi

@@ -224,6 +224,13 @@ or @code{beep} to report errors.
 An error message should start with a capital letter but should not end
 with a period.
 
+@item
+Many commands that take a long time to execute display a message that
+says @samp{Operating...} when they start, and change it to
+@samp{Operating...done} when they finish.  Please keep the style of
+these messages uniform: @emph{no} space around the ellipsis, and
+@emph{no} period at the end.
+
 @item
 Try to avoid using recursive edits.  Instead, do what the Rmail @kbd{e}
 command does: use a new local keymap that contains one command defined