util/blit: document params to util_blit_pixels_writemask()

src/gallium/auxiliary/util/u_blit.c

@@ -262,6 +262,10 @@ regions_overlap(int srcX0, int srcY0,
  * Copy pixel block from src surface to dst surface.
  * Overlapping regions are acceptable.
  * Flipping and stretching are supported.
+ * \param filter  one of PIPE_TEX_MIPFILTER_NEAREST/LINEAR
+ * \param writemask  controls which channels in the dest surface are sourced
+ *                   from the src surface.  Disabled channels are sourced
+ *                   from (0,0,0,1).
  * XXX what about clipping???
  * XXX need some control over blitting Z and/or stencil.
  */