[css-color-4] Add out of range clamping to the prose, as well as the pseudocode

Author: svgeesus

css-color-4/Overview.bs

@@ -5291,6 +5291,12 @@ Gamut Mapping
 	and the color difference formula used is <a href="#color-difference-OK">deltaEOK</a>.
 	The local-MINDE improvement is used.
 
+	For colors which are out of range on the Lightness axis,
+	white is returned in the destination color space
+	if the Lightness is greater than or equal to 1.0,
+	while black is returned in the destination color space
+	if the Lightness is less than or equal to 0.0.
+
 	For the binary search implementation,
 	at each step in the search,
 	the deltaEOK is computed between the current mapped color