theme: add documentation on group "terminology/tab_drag_thumb"

ChangeLog.theme

@@ -4,6 +4,8 @@ Terminology 1.7.0
 
 Changes since 1.6.0:
 --------------------
+  * Add group "terminology/tab_drag_thumb", used to have a thumbnail of a
+  terminal being dragged.
   * In group "terminology/tabbar_back", add signal "bell,off" from "terminology"
   to unmark a tab as having missed the bell.
   * In group "terminology/background", add signals "hdrag,on" (default) and

THEME.md

@@ -209,3 +209,19 @@ __TODO__
 
 # `terminology/mediactrl`
 __TODO__
+
+
+
+# `terminology/tab_drag_thumb`
+A thumbnail of a tab being dragged.
+
+## Swallowed parts
+### `terminology.content`
+Here is swallowed an object of group `terminology.background`.
+
+### `terminology.title`
+Title of the tab.
+
+## Signal received
+### `bell`
+To mark the tab as having missed a bell.