gallium/docs: add info about transfer boxes and array textures

src/gallium/docs/source/context.rst

@@ -354,6 +354,22 @@ Any pointers into the map should be considered invalid and discarded.
 Basically get_transfer, transfer_map, data write, transfer_unmap, and
 transfer_destroy all in one.
 
+
+The box parameter to some of these functions defines a 1D, 2D or 3D
+region of pixels.  This is self-explanatory for 1D, 2D and 3D texture
+targets.
+
+For PIPE_TEXTURE_1D_ARRAY, the box::y and box::height fields refer to the
+array dimension of the texture.
+
+For PIPE_TEXTURE_2D_ARRAY, the box::z and box::depth fields refer to the
+array dimension of the texture.
+
+For PIPE_TEXTURE_CUBE, the box:z and box::depth fields refer to the
+faces of the cube map (z + depth <= 6).
+
+
+
 .. _transfer_flush_region:
 
 transfer_flush_region