[TASK] Better example for FlashMessage finisher

- Show how to use messageArguments
- messageCode is mandatory (Don't know why, but yes it is)
--> As it is mandatory, it does not make sense to write `if needed`
- remove `Default value` from mandatory options
--> As all calls in TYPO3 Core are strict types, a value like `null`
will throw an Exception

Releases: master
Resolves: #88247
Change-Id: Ibf101a7c35ae01c975dbcaf162ff38f4eef27221
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/60631


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Jörg Bösche <typo3@joergboesche.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Jörg Bösche <typo3@joergboesche.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>