[css-color-4] Clarify column-major, link matrix library. Fix #6769

Author: svgeesus

css-color-4/Overview.bs

@@ -5199,7 +5199,9 @@ Default Style Rules</h2>
 
 <em>This section is not normative.</em>
 
-For clarity, a library is used for matrix multiplication.
+For clarity, <a href="multiply-matrices.js">a library</a> is used for matrix multiplication.
+(This is more readable than inlining all the multiplies and adds).
+The matrices are in <a href="https://www.scratchapixel.com/lessons/mathematics-physics-for-computer-graphics/geometry/row-major-vs-column-major-vector">column-major order</a>.
 
 <pre class="include-code lang-javascript">
 path: conversions.js