Documentation update for the tiddler macro

Including describing a new parameter that we'll add once the tests are
more up to date

js/macros/tiddler.js

@@ -1,6 +1,41 @@
 /*\
 title: js/macros/tiddler.js
 
+The tiddler macros transcludes another macro into the tiddler being rendered.
+
+Parameters:
+	target: the title of the tiddler to transclude
+	template: the title of the tiddler to use as a template for the transcluded tiddler
+	with: optional parameters to be substituted into the rendered tiddler
+
+The simplest case is to just supply a target tiddler:
+
+<<tiddler Foo>> or <<transclude target:Foo>>
+
+This will render the tiddler Foo within the current tiddler. If the tiddler Foo includes
+the view macro (or other macros that reference the fields of the current tiddler), then the
+fields of the tiddler Foo will be accessed.
+
+If you want to transclude the tiddler as a template, so that the fields referenced by the view
+macro are those of the tiddler doing the transcluding, then you can instead specify the tiddler
+as a template:
+
+<<tiddler template:Foo>>
+
+The effect is the same as the previous example: the text of the tiddler Foo is rendered. The
+difference is that the view macro will access the fields of the tiddler doing the transcluding.
+
+The `target` and `template` parameters may be combined:
+
+<<tiddler target:Foo template:Bar>>
+
+Here, the text of the tiddler `Bar` will be transcluded, with the macros within it accessing the fields
+of the tiddler `Foo`.
+
+Finally, the `with` parameter is used to substitute values for the special markers $1, $2, $3 etc. The
+substitutions are performed on the tiddler whose text is being rendered: either the tiddler named in
+the `template` parameter or, if that parameter is missing, the tiddler named in the `target` parameter.
+
 \*/
 (function(){