doc: correct tracingChannel.traceCallback()

tracingChannel.traceCallback() requires a callback otherwise it throws and
invalid argument error. As a result arguments are not optional.

Correct the documentation to reflect that arguments are not options.

Besides that correct description regarding signaling of errors.

Remove an unneeded null check in wrappedCallback() which can't happen because
it's validated that callback is of type function.

doc/api/diagnostics_channel.md

@@ -862,7 +862,7 @@ channels.tracePromise(async () => {
 });
 ```
 
-#### `tracingChannel.traceCallback(fn[, position[, context[, thisArg[, ...args]]]])`
+#### `tracingChannel.traceCallback(fn, position, context, thisArg, ...args)`
 
 <!-- YAML
 added:
@@ -874,23 +874,23 @@ added:
 
 * `fn` {Function} callback using function to wrap a trace around
 * `position` {number} Zero-indexed argument position of expected callback
-* `context` {Object} Shared object to correlate trace events through
+  (defaults to last argument if `undefined` is passed)
+* `context` {Object} Shared object to correlate trace events through (defaults
+  to `{}` if `undefined` is passed)
 * `thisArg` {any} The receiver to be used for the function call
-* `...args` {any} Optional arguments to pass to the function
+* `...args` {any} arguments to pass to the function (must include the callback)
 * Returns: {any} The return value of the given function
 
-Trace a callback-receiving function call. This will always produce a
+Trace a callback-receiving function call. The callback is expected to follow
+the error as first arg convention typically used. This will always produce a
 [`start` event][] and [`end` event][] around the synchronous portion of the
 function execution, and will produce a [`asyncStart` event][] and
 [`asyncEnd` event][] around the callback execution. It may also produce an
-[`error` event][] if the given function throws an error or the returned
-promise rejects. This will run the given function using
+[`error` event][] if the given function throws or the first argument passed to
+the calblack is set. This will run the given function using
 [`channel.runStores(context, ...)`][] on the `start` channel which ensures all
 events should have any bound stores set to match this trace context.
 
-The `position` will be -1 by default to indicate the final argument should
-be used as the callback.
-
 ```mjs
 import diagnostics_channel from 'node:diagnostics_channel';
 

lib/diagnostics_channel.js

@@ -371,9 +371,7 @@ class TracingChannel {
       // Using runStores here enables manual context failure recovery
       asyncStart.runStores(context, () => {
         try {
-          if (callback) {
-            return ReflectApply(callback, this, arguments);
-          }
+          return ReflectApply(callback, this, arguments);
         } finally {
           asyncEnd.publish(context);
         }