add note about sts region to doc
add note about sts region to doc

hartsock/vault,joelthompson/vault,hashicorp/vault,hashicorp/vault,hartsock/vault,hashicorp/vault,rnaveiras/vault,rnaveiras/vault,hashicorp/vault,quixoten/vault,hartsock/vault,hashicorp/vault,quixoten/vault,hashicorp/vault,kingland/vault,joelthompson/vault,rnaveiras/vault,kingland/vault,hartsock/vault,rnaveiras/vault,joelthompson/vault,joelthompson/vault,rnaveiras/vault,kingland/vault,kingland/vault,quixoten/vault,hartsock/vault,kingland/vault,quixoten/vault