doc: update assert.throws() examples

This updates two outdated examples to the current implementation.

PR-URL: #28263
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/assert.md

@@ -1188,10 +1188,15 @@ assert.throws(
 assert.throws(
   () => {
     const otherErr = new Error('Not found');
-    otherErr.code = 404;
+    // Copy all enumerable properties from `err` to `otherErr`.
+    for (const [key, value] of Object.entries(err)) {
+      otherErr[key] = value;
+    }
     throw otherErr;
   },
-  err // This tests for `message`, `name` and `code`.
+  // The error's `message` and `name` properties will also be checked when using
+  // an error as validation object.
+  err
 );
 ```
 
@@ -1234,9 +1239,10 @@ assert.throws(
     assert(err instanceof Error);
     assert(/value/.test(err));
     // Returning anything from validation functions besides `true` is not
-    // recommended. Doing so results in the caught error being thrown again.
-    // That is usually not the desired outcome. Throw an error about the
-    // specific validation that failed instead (as done in this example).
+    // recommended. By doing that, it's not clear what part of the validation
+    // failed. Instead, throw an error about the specific validation that failed
+    // (as done in this example) and add as much helpful debugging information
+    // to that error as possible.
     return true;
   },
   'unexpected error'
@@ -1278,11 +1284,9 @@ assert.throws(notThrowing, 'Second');
 // It does not throw because the error messages match.
 assert.throws(throwingSecond, /Second$/);
 
-// If the error message does not match, the error from within the function is
-// not caught.
+// If the error message does not match, an AssertionError is thrown.
 assert.throws(throwingFirst, /Second$/);
-// Error: First
-//     at throwingFirst (repl:2:9)
+// AssertionError [ERR_ASSERTION]
 ```
 
 Due to the confusing notation, it is recommended not to use a string as the