Simplify new Taglet API

The new Taglet API added as part of the new Doclet API has two similar .toString methods, one takes a DocTree and the other takes a List<DocTree>. The documentation does not explain when each might be called (which is a bug in itself) but the fact is, we don't need both: we can eliminate the one that takes a single DocTree and always/only use the one taking a List<DocTree>.

In addition, we should clarify the use of this method.

The API is currently:


    /**
     * Given the {@link DocTree DocTree} representation of this custom
     * tag, return its string representation, which is output
     * to the generated page.
     * @param tag the <code>Tag</code> representation of this custom tag.
     * @return the string representation of this <code>Tag</code>.
     */
    String toString(DocTree tag);

    /**
     * Given a List of {@link DocTree DocTrees} representing this custom
     * tag, return its string representation, which is output
     * to the generated page. This method should
     * return null if this taglet represents an inline or body tag.
     * @param tags the list of <code>DocTree</code>s representing this custom tag.
     * @return the string representation of this <code>Tag</code>.
     */
    String toString(List<? extends DocTree> tags);