Update documentation on how aggregates are handled

[geek-boy]

Add to documentation about how aggregates are handled as per https://help.sumologic.com/APIs/Search-Job-API/About-the-Search-Job-API#more-about-results:

If the query contains an aggregation, for example, count by _sourceCategory, then the messages are returned along with records resulting from the aggregation (similar to what a SQL database would return).