[doc] Std.is returns false for @:generic types (closes #7914)

std/Std.hx

@@ -29,6 +29,8 @@
 extern class Std {
 	/**
 		Tells if a value `v` is of the type `t`. Returns `false` if `v` or `t` are null.
+
+		If `t` is a class or interface with `@:generic` meta, the result is `false`.
 	**/
 	public static function is(v:Dynamic, t:Dynamic):Bool;