add WidgetVariable documentation

editions/tw5.com/tiddlers/wikitext/Variables in WikiText.tid

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+created: 20141002141231992
+modified: 20141002144153170
+tags: WikiText
+title: Variables in WikiText
+type: text/vnd.tiddlywiki
+caption: Hard Linebreaks
+! Defining Variables
+
+Variables contains values defined by [[Widgets]].
+Variables are available within the widget that defines them, and the child widgets in the widget tree.
+
+Variables are defined by:
+
+* TiddlyWiki core and viewtemplate
+* SetWidget
+* ListWidget
+* [[Macro definition|Macros in WikiText]]
+
+! Using Variables
+
+Variables are used with:
+
+* Variable substitution `$(name)$`
+* Concise shortcut  `<<name>>`
+* [[Filter expression|Introduction to Filters]] `[operator<variable-operand>]`
+
+Built-in [[variables|Variables]] are useful for dynamic WikiText.