Rename decoration_border to child_border.

fixes #2154

docs/userguide

@@ -836,9 +836,9 @@ workspace "2: vim" output VGA1
 You can change all colors which i3 uses to draw the window decorations.
 
 *Syntax*:
--------------------------------------------------------------------------
-<colorclass> <border> <background> <text> <indicator> <decoration_border>
--------------------------------------------------------------------------
+--------------------------------------------------------------------
+<colorclass> <border> <background> <text> <indicator> <child_border>
+--------------------------------------------------------------------
 
 Where colorclass can be one of:
 
@@ -863,8 +863,8 @@ client.background::
 Colors are in HTML hex format (#rrggbb), see the following example:
 
 *Examples (default colors)*:
----------------------------------------------------------
-# class                 border  backgr. text    indicator decoration_border
+----------------------------------------------------------------------
+# class                 border  backgr. text    indicator child_border
 client.focused          #4c7899 #285577 #ffffff #2e9ef4   #285577
 client.focused_inactive #333333 #5f676a #ffffff #484e50   #5f676a
 client.unfocused        #333333 #222222 #888888 #292d2e   #222222
@@ -872,10 +872,10 @@ client.urgent           #2f343a #900000 #ffffff #900000   #900000
 client.placeholder      #000000 #0c0c0c #ffffff #000000   #0c0c0c
 
 client.background       #ffffff
----------------------------------------------------------
+----------------------------------------------------------------------
 
 Note that for the window decorations, the color around the child window is the
-"decoration_border", and "border" color is only the two thin lines around the
+"child_border", and "border" color is only the two thin lines around the
 titlebar.
 
 The indicator color is used for indicating where a new window will be opened.