GROOVY-6424: @GrabExclude's group element is documented as optional but the implementation requires it
GROOVY-6424: @GrabExclude's group element is documented as optional but the implementation requires it

improved the javadoc and added some tests
GROOVY-6424: @GrabExclude's group element is documented as optional but the implementation requires it

added some cosmetic enhancements to the javadoc