doc: improve .throws RegExp info

It was not clear why the error name is actually also tested for when
using a regular expression. This is now clarified.

Backport-PR-URL: #19230
PR-URL: #17585
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Evan Lucas <[email protected]>

Author: BridgeAR

doc/api/assert.md

@@ -738,12 +738,15 @@ assert.throws(
 
 Validate error message using [`RegExp`][]:
 
+Using a regular expression runs `.toString` on the error object, and will
+therefore also include the error name.
+
 ```js
 assert.throws(
   () => {
     throw new Error('Wrong value');
   },
-  /value/
+  /^Error: Wrong value$/
 );
 ```