documented the new behavior in man page

dwm.1

@@ -62,6 +62,9 @@ Focus previous window.
 .B Mod1-Return
 Zooms/cycles current window to/from master area (tiling mode), toggles maximization current window (floating mode).
 .TP
+.B Mod1-b
+Toggle stacking area position (tiling mode only).
+.TP
 .B Mod1-g
 Grow current area (tiling mode only).
 .TP