Clarify role of @PostConstruct annotated methods in validation steps

The first two validation steps are

* Phase 1 Inject field values and initialize bean properties as described in Section 3.2.
* Phase 2 Validate annotations on fields, property getters (if enabled) and the resource class. The order in which these validations are executed is implementation dependent.

In the case that CDI is enabled, RESTEasy currently initiates field/property validation at the end of InjectionTarget.inject(). However, since further initialization can take place in a @PostConstruct annotated method, field/property validation should be initiated from InjectionTarget.postConstruct().

It would be helpful if the role played by @PostConstruct methods were mentioned explicitly in the specification.