wibox.layout.fixed: Update the property documentation.

lib/wibox/layout/fixed.lua

@@ -299,7 +299,7 @@ end
 --@DOC_wibox_layout_fixed_spacing_widget_EXAMPLE@
 --
 -- @property spacing_widget
--- @tparam widget spacing_widget
+-- @tparam[opt=nil] widget|nil spacing_widget
 -- @propemits true false
 -- @interface layout
 
@@ -404,7 +404,7 @@ end
 -- widget will get all the space that is left. If this is false, the last widget
 -- won't be handled specially and there can be space left unused.
 -- @property fill_space
--- @tparam boolean fill_space
+-- @tparam[opt=false] boolean fill_space
 -- @propemits true false
 
 function fixed:fill_space(val)
@@ -455,7 +455,8 @@ end
 --@DOC_wibox_layout_fixed_spacing_EXAMPLE@
 --
 -- @property spacing
--- @tparam number spacing Spacing between widgets.
+-- @tparam[opt=0] number spacing Spacing between widgets.
+-- @negativeallowed true
 -- @propemits true false
 -- @interface layout