Improve handling of @inheritDoc

The only taglet that along with its own tag needs to know the immediately enclosing tag is InheritDocTaglet:

    @return {@inheritDoc}
    @throws NullPointerException {@inheritDoc}
    @param p {@inheritDoc}

However, the immediately enclosing tag is unconditionally passed to all taglets. If we stop passing it and make InheritDocTaglet compute it instead, the code becomes cleaner.