function acosh( x ) {
	return addon( x );
}
Computes the hyperbolic arcsine of a number.

@private
@param {number} x - input value
@returns {number} hyperbolic arccosine

@example
var v = acosh( 1.0 );
// returns 0.0

@example
var v = acosh( 2.0 );
// returns ~1.317

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