Streamline custom validator examples

[DanielSiepmann]

Raised by @maikusch, the sections within Documentation/9-CrosscuttingConcerns/2-validating-domain-objects.rst are misleading. Code examples were adjusted via d9509fc#diff-eaedc983c891f372262095aee2860fc6 but do no longer match. validate() is now implemented instead of isValid(), but this does not match to extending the AbstractValidator class.

Which way is preferred and why. This way should be used through the whole section, with a small side note why it is preferred over the other.

[linawolf]

Done and moved to typo3 explained