Clarify when custom validation is triggered

[evanw555]

It's apparently not clear to many customers when custom validation (i.e. "validate" PDL annotation) is triggered. Many customers seem to believe that these validations should be triggered on template initialization or on template getters/setters, which isn't true.

We should update the validation documentation to make this clear: https://linkedin.github.io/rest.li/Validation-in-Rest_li#custom-validation-rules