function cdf( x, sigma ) {
	return addon( x, sigma );
}
Evaluates the cumulative distribution function (CDF) for a Rayleigh distribution with scale parameter `sigma` at a value `x`.

@private
@param {number} x - input value
@param {NonNegativeNumber} sigma - scale parameter
@returns {Probability} evaluated CDF

@example
var y = cdf( 2.0, 3.0 );
// returns ~0.199

@example
var y = cdf( 1.0, 2.0 );
// returns ~0.118

@example
var y = cdf( -1.0, 4.0 );
// returns 0.0

@example
var y = cdf( NaN, 1.0 );
// returns NaN

@example
var y = cdf( 0.0, NaN );
// returns NaN

@example
// Negative scale parameter:
var y = cdf( 2.0, -1.0 );
// returns NaN