function sqrt1pm1( x ) {
	return addon( x );
}
Computes the value of `sqrt(1+x)-1`.

@private
@param {number} x - input value
@returns {number} square root of `1+x` minus one

@example
var v = sqrt1pm1( 3.0 );
// returns 1.0

@example
var v = sqrt1pm1( 0.5 );
// returns ~0.225

@example
var v = sqrt1pm1( 0.02 );
// returns ~0.01

@example
var v = sqrt1pm1( -0.5 );
// returns ~-0.293

@example
var v = sqrt1pm1( -1.1 );
// returns NaN

@example
var v = sqrt1pm1( NaN );
// returns NaN