Aggregates text from child nodes for fulltext queries.
Example: let's say node /x is of type 'web page', but the actual content is
stored in child nodes; say /x/section1 contains "Hello" and /x/section2
contains "World". If index aggregation is configured correctly, it will
combine all the text of the child nodes, and index that as /x. When doing a
fulltext search for for "Hello World", the index will then return /x.