function upper( x, n, alpha ) {
	return ( x === n ) ? 1 : betaQuantile( 1 - alpha, x + 1, n - x );
}
Calculates the upper endpoint of a confidence interval.

@private
@param {NonNegativeInteger} x - number of successes
@param {NonNegativeInteger} n - total number of observations
@param {number} alpha - significance level
@returns {number} upper endpoint