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.

PR-URL: https://github.com/nodejs/node/pull/17585
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>

doc/api/assert.md

@@ -740,12 +740,15 @@ 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$/
 );
 ```