mqa.rst: document render_bitdepth field

mqa.rst

@@ -222,10 +222,9 @@ information for the decoder. ::
           orig_rate                               uint(5)
           src_rate                                uint(5)
           render_filter                           uint(5)
-          unknown                                 uint(5)
-          unknown                                 uint(1)
-          unknown                                 uint(1)
-          unused                                  bits(1)
+          unknown                                 bits(2)
+          render_bitdepth                         uint(2)
+          unknown                                 bits(4)
           auth_info                               uint(4)
           auth_level                              uint(4)
           item_count                              uint(7)
@@ -270,6 +269,18 @@ information for the decoder. ::
 `render_filter`
   Index of upsampling filter to use in the render stage.
 
+`render_bitdepth`
+  Index of bit depth of dither noise in the render stage.
+
+  =====  =========
+  Index  Bit depth
+  =====  =========
+  0      20
+  1      18
+  2      16
+  3      15
+  =====  =========
+
 `auth_info`
   Unknown meaning.  Also present in encrypted `auth_data`.