Validate web-api constructs

FilterDetails, ServletDetails and other interfaces have sorely under-documented attributes, like urlPatterns. Their semantics are related to the servlet spec, but that connection is not explicitly stated and, worse, validity of attribute values is not checked at all.

Thoroughly document each attribute, including links to their semantics and allowed value formats, as most of them are plain Strings.

Also make sure values are checked during build-time – either by turning interfaces into abstract classes with a Value.Check annotation, or by ditching immutables.org completely.