ValidateBy / buildMessage is a much cleaner interface (should be preferred in docs)

[devin-fee-ah]

I couldn't quite get what I wanted out of the Custom Validation Decorators docs, so I went to IsString and found a much more helpful pattern. I think this should be the preferred approach.

Specifically, I'm talking about buildMessage and ValidateBy.

[Arnaud-Dev-Nodejs]

I approve the change of the documentation. 👍 ValidateBy is cleaner and easier to understand.

[jtlapp]

I just did the same thing. In fact, I was ready to abandon class-validator because I wasn't getting type checking on my custom constraints. I spent some time looking at other validation packages before wondering why I couldn't just create a first-class validator here. Turns out you can!