Allow Javadoc to reference imports

These style errors continue to creep in due to IDE misconfiguration so
we might as well stop checking for it.
This commit allows Javadoc to reference imports, i.e., partially disables the check.  I prefer the existing behavior, i.e., warning about imports only referenced by Javadoc.  The Checkstyle manual also suggests this:

> Optionally: it is referenced in Javadoc comments. This check is off by default, as it is considered bad practice to introduce a compile time dependency for documentation purposes only. As an example, the import java.util.Date would be considered referenced with the Javadoc comment {@link Date}. The alternative to avoid introducing a compile time dependency would be to write the Javadoc comment as {@link java.util.Date}.

However, I do not know which IDE configuration to recommend to stop people from introducing these violations.