Update link widget docs

editions/tw5.com/tiddlers/widgets/LinkWidget.tid

@@ -11,11 +11,7 @@ The `link` widget generates links to tiddlers.
 
 The content of the link widget is rendered within the `<a>` tag.
 
-The default value of the tooltip attribute is `<<tw-wikilink-tooltip>>`. If that variable is not defined then the following text is used:
+The default value of the tooltip attribute is `<<tw-wikilink-tooltip>>`.
-
-```
-<$transclude field="tooltip"><$transclude field="title"/></$transclude>
-```
 
 This means that you can control the text of a link tooltip in several ways:
 
@@ -38,6 +34,17 @@ Renders as:
 
 Note that the tooltip is rendered with the current tiddler set to the target of the link.
 
+A useful convention is to set the tooltip like this:
+
+```
+\define tw-wikilink-tooltip()
+<$transclude field="tooltip"><$transclude field="title"/></$transclude>
+\end
+```
+
+This causes the tooltip to be the ''tooltip'' field of the target tiddler. If the field isn't present, then the title is used instead.
+
+
 ! CSS Classes
 
 * `tw-tiddlylink` - applied to all links