From at least Node 4.2 the documentation says:

The first line of the trace, instead of being prefixed with ErrorType: message, will be the result of calling targetObject.toString().

This test program:

var assert = require('assert');

function X(msg) {
  this.message = msg;
}
X.prototype.name = 'X';
X.prototype.toString = function() { return 'toString'; }

var x = new X('something');
assert.deepEqual(x, {message: 'something'});
assert.equal(x.toString(), 'toString');

Error.captureStackTrace(x);
assert.equal(x.stack.split('\n')[0], 'X: something');

proves the description incorrect. The first line is produced by deps/v8/src/messages.js in ErrorToStringDetectCycle from the name and message properties of targetObject with this logic:

if (name === "") return message;
if (message === "") return name;
return name + ": " + message;

where name defaults to "Error" and message defaults to "".

I'm not good at user-facing technical documentation so I'd rather not try to craft replacement text for something that complex.