document platform inconsistencies on "".split (closes #2009)

std/String.hx

@@ -104,6 +104,9 @@ extern class String {
 	/**
 		Splits `this` String at each occurence of `delimiter`.
 		
+		If `this` String is the empty String "", the result is not consistent
+		across targets and may either be `[]` (on Js, Cpp) or `[""]`.
+		
 		If `delimiter` is the empty String "", `this` String is split into an
 		Array of `this.length` elements, where the elements correspond to the
 		characters of `this` String.