FTS: Add example of multiple-index alias to documentation

Please add description and example of using FTS aliases to allow for search against multiple indexes. Existing documentation (at https://docs.couchbase.com/server/6.0/fts/fts-creating-indexes.html#using-index-aliases ) highlights only administrative switching of one index to another. Using the the alias to search against multiple indexes is a powerful feature and our customers are often unaware of it.

Thanks for your help!