The terms of any writing which is in existence when a will or trust instrument is executed, including but not limited to a statement of administrative provisions and fiduciary powers recorded in a record office of this State, may be incorporated into the will or trust instrument by reference to it to the extent the language of the will or trust instrument manifests an intent to do so and describes the writing sufficiently to permit its identification. Nothing in this section shall be construed as casting doubt upon the validity of incorporation by reference made prior to the adoption of this section.