ipc: document scratchpad_state (#4466)

fixes i3/i3#4465

RELEASE-NOTES-next

@@ -29,6 +29,7 @@ option is enabled and only then sets a screenshot as background.
  └────────────────────────────┘
 
   • default config: use dex for XDG autostart
+  • docs/ipc: document scratchpad_state
   • i3-nagbar: position on focused monitor by default
   • i3-nagbar: add option to position on primary monitor
   • alternate focusing tab/stack children-parent containers by clicking on their titlebars

docs/ipc

@@ -450,12 +450,15 @@ fullscreen_mode (integer)::
 floating (string)::
 	Floating state of container.
 	Can be either "auto_on", "auto_off", "user_on" or "user_off"
-
 nodes (array of node)::
 	The tiling (i.e. non-floating) child containers of this node.
 floating_nodes (array of node)::
 	The floating child containers of this node. Only non-empty on nodes with
 	type +workspace+.
+scratchpad_state (string)::
+	Whether the window is not in the scratchpad ("none"), freshly moved to
+	the scratchpad but not yet resized ("fresh") or moved to the scratchpad
+	and resized ("changed").
 
 Please note that in the following example, I have left out some keys/values
 which are not relevant for the type of the node. Otherwise, the example would