[css-values-5] Clarify that stepped values are uniformly random.

Author: tabatkins

css-values-5/Overview.bs

@@ -575,7 +575,9 @@ Generating a Random Numeric Value: the ''random()'' function</h3>
 			specifies a step value:
 			the values the function can resolve to
 			are further restricted to the form <code>min + (N * step)</code>,
-			where N is a non-negative integer.
+			where N is a non-negative integer
+			chosen uniformly randomly from the possible values
+			that result in an in-range value.
 
 			<div class=example>
 				For example, ''random(100px, 300px, by 50px)''
@@ -597,7 +599,6 @@ Generating a Random Numeric Value: the ''random()'' function</h3>
 				To be safe, you can put the maximum value
 				<em>slightly above</em> where you expect the final step to land,
 				like ''random(100px, 201px, by 100px / 3)''.
-
 			</div>
 
 			<div class=example>
@@ -611,6 +612,15 @@ Generating a Random Numeric Value: the ''random()'' function</h3>
 				is restricted to resolving only to integers;
 				etc.
 			</div>
+
+			Note: The definition of the step <em>does not</em> allow
+			for naively generating a random value in the range
+			and then rounding it to the nearest step value,
+			as that can result in the values not appearing with the same weights.
+			For example, ''random(100px, 200px, by 50px)''
+			has to generate the three possible values each with a 1/3 chance;
+			a naive rounding-based method will instead incorrectly generate ''150px''
+			twice as often as the boundary values.
 	</dl>
 
 	All of the [=calculation=] arguments can resolve to any <<number>>, <<dimension>>, or <<percentage>>,
@@ -650,6 +660,10 @@ Argument Ranges</h4>
 	If C is infinite,
 	the result is A.
 
+	(If C is zero or negative,
+	the result is A,
+	but that falls out of the standard definition.)
+
 	Note: As usual for [=math functions=],
 	if any argument calculation is NaN,
 	the result is NaN.