Fixed color-mix description

Clarified what happens when the specified percentages add up to less than 100%.

files/en-us/web/css/color_value/color-mix/index.md

@@ -56,6 +56,7 @@ Functional notation: `color-mix(<color-interpolation-method>, <color>[<percentag
     - If `p2` is omitted, then `p2 = 100% - p1`.
     - If `p1 = p2 = 0%`, the function is invalid.
     - If `p1 + p2 ≠ 100%`, then `p1' = p1 / (p1 + p2)` and `p2' = p2 / (p1 + p2)`, where `p1'` and `p2'` are the normalization results.
+      - If `p1 + p2 < 100%`, then an alpha multiplier of `p1 + p2` is applied to the resulting color. This is similar to mixing in [`transparent`](/en-US/docs/Web/CSS/named-color#transparent), with percentage `pt = 100% - p1 - p2`.
 
 ### Formal syntax