TJ doc: Density params require YCbCr or grayscale

Since libjpeg-turbo does not support Exif, the only way it can embed
density information in a JPEG image is by using the JFIF marker, which
is only written if the JPEG colorspace is YCbCr or grayscale.
(Referring to the conversation under #793, we may need to further
restrict that to 8-bit-per-sample JPEG images, because the JFIF spec
requires 8-bit data precision.)

turbojpeg.h

@@ -836,6 +836,9 @@ enum TJPARAM {
    * Windows BMP file by #tj3SaveImage8() if the value of #TJPARAM_DENSITYUNITS
    * is `2`.
    *
+   * This parameter has no effect unless the JPEG colorspace (see
+   * #TJPARAM_COLORSPACE) is #TJCS_YCbCr or #TJCS_GRAY.
+   *
    * @see TJPARAM_DENSITYUNITS
    */
   TJPARAM_XDENSITY,
@@ -853,6 +856,9 @@ enum TJPARAM {
    * Windows BMP file by #tj3SaveImage8() if the value of #TJPARAM_DENSITYUNITS
    * is `2`.
    *
+   * This parameter has no effect unless the JPEG colorspace (see
+   * #TJPARAM_COLORSPACE) is #TJCS_YCbCr or #TJCS_GRAY.
+   *
    * @see TJPARAM_DENSITYUNITS
    */
   TJPARAM_YDENSITY,
@@ -874,6 +880,9 @@ enum TJPARAM {
    * density information, and the value of this parameter is stored to a
    * Windows BMP file by #tj3SaveImage8() if the value is `2`.
    *
+   * This parameter has no effect unless the JPEG colorspace (see
+   * #TJPARAM_COLORSPACE) is #TJCS_YCbCr or #TJCS_GRAY.
+   *
    * @see TJPARAM_XDENSITY, TJPARAM_YDENSITY
    */
   TJPARAM_DENSITYUNITS,