LRDOCS-990 Add Javadoc for searchJournalArticle methods

Hi Tina,

For Javadoc Thursday, I'm in the process of adding Javadoc to the JournalArticleLocalServiceImpl class to document new methods.

I've added several new method descriptions for methods that you introduced (from this [commit](https://github.com/liferay/liferay-portal/commit/5ae57b3c388fa3607fb1df694a95ccc8569ce418)), and request you to review them to make sure they're accurate. I've modeled the Javadoc to stay consistent with other Javadoc in the class.

Please let me know if anything in particular should be added to the searchJournalArticles() methods' Javadoc that would be important or if there are any inaccuracies. Feel free to edit the Javadoc directly and send a PR back to me, if it's more convenient. I've added three method descriptions in total.

Thanks!