For the underlying JSR303 Bean Validation specification, checkout the JCP website.
The complete set of online JavaDocs for the provider and optional modules can be found online or downloaded from a Maven repository.
API Docs for 0.3-incubating
API Docs for 0.2-incubating
API Docs for 0.1-incubating
Since Apache BVal implements the JSR303 Bean Validation specification, most articles and HowTo guides for the RI (Hibernate Validator) can also be used as a reference.