filter dialog: add tooltip re: filter syntax

classes/pref/filters.php

@@ -980,9 +980,14 @@ class Pref_Filters extends Handler_Protected {
 
 		print "<input dojoType=\"dijit.form.ValidationTextBox\"
 			 required=\"true\" id=\"filterDlg_regExp\"
+			 placeholder=\"match|keywords\"
 			 style=\"font-size : 16px; width : 20em;\"
 			 name=\"reg_exp\" value=\"$reg_exp\"/>";
 
+		print "<div dojoType=\"dijit.Tooltip\" connectId=\"filterDlg_regExp\" position=\"below\">
+			".__("Regular expression, without outer delimiters (i.e. slashes)")."
+		</div>";
+
 		print "<hr/>";
 		print "<input id=\"filterDlg_inverse\" dojoType=\"dijit.form.CheckBox\"
 			 name=\"inverse\" $inverse_checked/>";