doc: update assert's validation functions

Using `assert.throws()` or `assert.rejects()` in combination with
validation function can be very powerful. Using them by returning
any value besides `true` makes debugging very difficult though
because there's no context or reason why the error validation failed.
Thus, it is recommended to throw an error in such cases instead of
returning anything besides `true`.

PR-URL: https://github.com/nodejs/node/pull/27781


Reviewed-By: Rich Trott <rtrott@gmail.com>