Why does typedoc ignore my interface declaration?

type S=string|undefined;
declare global {
    export interface String {
        /**
         * returns string between borders
         * @param   borderA - left border
         * @param   borderB - right border
         * @param aIsFirst - true if borderA is first, false if is last
         * @param bIsFirst - true if borderB is first, false if is last
         */
        between(borderA:S, borderB:S, aIsFirst:boolean, bIsFirst:boolean):S;
    }
}

typedoc command: typedoc --out ./documentation/ src/

It generates the folder with HTML, taken from README.md, but no traces of my interface is ever found there...