Update docs for $:/tags/SideBarSegment (#3579)

The possibility to rearrange stuff "inside" tag pills is not obvious so makes sense to mention.

editions/tw5.com/tiddlers/systemtags/SystemTag_ $__tags_SideBarSegment.tid

@@ -6,4 +6,6 @@ tags: SystemTags
 title: SystemTag: $:/tags/SideBarSegment
 type: text/vnd.tiddlywiki
 
-The [[system tag|SystemTags]] `$:/tags/SideBarSegment` marks sidebar segments
+The [[system tag|SystemTags]] `$:/tags/SideBarSegment` marks sidebar segments
+
+Note: You can rearrange the segments by drag-and-dropping the titles inside the tag pill <<tag $:/tags/SideBarSegment>>