userDoc( alias, [ref,] doc )
    Adds user-defined documentation.

    If user-defined documentation already exists for the provided alias, the
    current documentation is overwritten.

    Parameters
    ----------
    alias: string
        Documentation alias.

    ref: any (optional)
        Object reference.

    doc: string
        Documentation.

    Examples
    --------
    > function foo() {};
    > userDoc( 'foo', foo, '\nfoo()\n    Foo bar.\n' );
    > help( foo )

    See Also
    --------
    help