function gamma1pm1( x ) {
	return addon( x );
}
Computes `gamma(x+1) - 1`.

@private
@param {number} x - input value
@returns {number} function value

@example
var v = gamma1pm1( 0.2 );
// returns ~-0.082

@example
var v = gamma1pm1( -9.2 );
// returns ~-1.0

@example
var v = gamma1pm1( 0.0 );
// returns 0.0

@example
var v = gamma1pm1( -3.0 );
// returns NaN

@example
var v = gamma1pm1( NaN );
// returns NaN