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.)

java/doc/org/libjpegturbo/turbojpeg/TJ.html

@@ -1878,7 +1878,10 @@ extends java.lang.Object</pre>
  </ul>
 
  <p>This value is stored in or read from the JPEG header.  It does not
- affect the contents of the JPEG image.</div>
+ affect the contents of the JPEG image.
+
+ <p>This parameter has no effect unless the JPEG colorspace (see
+ <a href="#PARAM_COLORSPACE"><code>PARAM_COLORSPACE</code></a>) is <a href="#CS_YCbCr"><code>CS_YCbCr</code></a> or <a href="#CS_GRAY"><code>CS_GRAY</code></a>.</div>
 <dl>
 <dt><span class="seeLabel">See Also:</span></dt>
 <dd><a href="#PARAM_DENSITYUNITS"><code>PARAM_DENSITYUNITS</code></a>, 
@@ -1903,7 +1906,10 @@ extends java.lang.Object</pre>
  </ul>
 
  <p>This value is stored in or read from the JPEG header.  It does not
- affect the contents of the JPEG image.</div>
+ affect the contents of the JPEG image.
+
+ <p>This parameter has no effect unless the JPEG colorspace (see
+ <a href="#PARAM_COLORSPACE"><code>PARAM_COLORSPACE</code></a>) is <a href="#CS_YCbCr"><code>CS_YCbCr</code></a> or <a href="#CS_GRAY"><code>CS_GRAY</code></a>.</div>
 <dl>
 <dt><span class="seeLabel">See Also:</span></dt>
 <dd><a href="#PARAM_DENSITYUNITS"><code>PARAM_DENSITYUNITS</code></a>, 
@@ -1933,7 +1939,10 @@ extends java.lang.Object</pre>
  </ul>
 
  <p>This value is stored in or read from the JPEG header.  It does not
- affect the contents of the JPEG image.</div>
+ affect the contents of the JPEG image.
+
+ <p>This parameter has no effect unless the JPEG colorspace (see
+ <a href="#PARAM_COLORSPACE"><code>PARAM_COLORSPACE</code></a>) is <a href="#CS_YCbCr"><code>CS_YCbCr</code></a> or <a href="#CS_GRAY"><code>CS_GRAY</code></a>.</div>
 <dl>
 <dt><span class="seeLabel">See Also:</span></dt>
 <dd><a href="#PARAM_XDENSITY"><code>PARAM_XDENSITY</code></a>,