For the underlying JSR303 Bean Validation specification, checkout the JCP website.
TBD...
The complete set of online JavaDocs for the provider and optional modules can be found online or downloaded from a Maven repository.
Online 0.3-incubating JavaDocs
You can also download the built JavaDocs from the Maven repository - bval-jsr303-0.3-incubating-javadoc.jar bval-core-0.3-incubating-javadoc.jar
Online 0.2-incubating JavaDocs
You can also download the built JavaDocs from the Maven repository - bval-jsr303-0.2-incubating-javadoc.jar bval-core-0.2-incubating-javadoc.jar
Online 0.1-incubating JavaDocs
You can also download the built JavaDocs from the Maven repository - bval-jsr303-0.1-incubating-javadoc.jar bval-core-0.1-incubating-javadoc.jar
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: