Bug 1263962 - Expand on the StyleAnimationValue::ExtractComputedValue documentation. r=dholbert

layout/style/StyleAnimationValue.h

@@ -226,6 +226,11 @@ public:
    * Gets the computed value for the given property from the given style
    * context.
    *
+   * Obtaining the computed value allows us to animate properties when the
+   * content author has specified a value like "inherit" or "initial" or some
+   * other keyword that isn't directly interpolatable, but which *computes* to
+   * something interpolatable.
+   *
    * @param aProperty     The property whose value we're looking up.
    * @param aStyleContext The style context to check for the computed value.
    * @param [out] aComputedValue The resulting computed value.