better doc for UnicodeString

std/UnicodeString.hx

@@ -26,9 +26,14 @@ import haxe.iterators.StringIteratorUnicode;
 import haxe.iterators.StringKeyValueIteratorUnicode;
 
 /**
-	This abstract provides consistent cross-target unicode support.
+	This abstract provides consistent cross-target unicode support for characters of any width.
 
-	@see https://haxe.org/manual/std-UnicodeString.html
+	Due to differing internal representations of strings across targets, only the basic
+	multilingual plane (BMP) is supported consistently by `String` class.
+
+	This abstract provides API to consistently handle all characters even beyond BMP.
+
+	@see https://haxe.org/manual/std-String-unicode.html
 **/
 @:forward
 @:access(StringTools)