function logcdf( x, sigma ) {
	return addon( x, sigma );
}
Evaluates the logarithm of 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 {number} evaluated logCDF

@example
var y = logcdf( 2.0, 3.0 );
// returns ~-1.613

@example
var y = logcdf( 1.0, 2.0 );
// returns ~-2.141

@example
var y = logcdf( -1.0, 4.0 );
// returns -Infinity

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

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

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