function pdf( x, sigma ) {
	return addon( x, sigma );
}
Evaluates the probability density function (PDF) 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 PDF

@example
var y = pdf( 0.3, 1.0 );
// returns ~0.287

@example
var y = pdf( 2.0, 0.8 );
// returns ~0.137

@example
var y = pdf( -1.0, 0.5 );
// returns 0.0

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

@example
var y = pdf( NaN, 2.0 );
// returns NaN

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