Update section on AbstractValidator to put emphasis on interface

[sypets]

Alexander Schnitzler:

This PR is right since the code example shows a validator that inherits AbstractValidator. While this change is right, I'd like to see a change in the overall docs in the future, showing that the ValidatorInterface is the actual important API.

comment on PR #232

[sypets]

Issues is older than 3 years, low priority.