formalize attributes

luigi/widget/progress.lua

@@ -1,3 +1,12 @@
+--[[--
+A progress bar.
+
+Set the widget's `value` property to a decimal value
+between 0 and 1 (inclusive) to change the width of the bar.
+
+@widget progress
+--]]--
+
 return function (self)
     self.value = 0
     self.flow = 'x' -- TODO: support vertical progress?