[css-transforms-2] Mention matrix for rotate3d() assumes x, y, and z have been normalized.

Author: dbaron

css-transforms-2/Overview.bs

@@ -1334,6 +1334,12 @@ One translation unit on a matrix is equivalent to 1 pixel in the local coordinat
 
 		<img src="images/rotate3dvariables.png" alt="\newline sc = \sin (\alpha/2) \cdot \cos (\alpha/2) \newline sq = \sin^2 (\alpha/2)" width="221" height="50">
 
+                        and where x, y, and z have been normalized
+                        (that is,
+                        where the x, y, and z values given
+                        have been divided by
+                        the square root of the sum of their squares).
+
 	<li id="PerspectiveDefined">
 			A perspective projection matrix with the parameter <em>d</em> is equivalent to the matrix: