Depending on the search that you save as an alert, the custom alert action might create multiple events or incidents in ServiceNow. This can occur if the search string that you save as an alert returns multiple events. The number of events returned by the search equals the number of incidents or events created in ServiceNow.
I have tested with a multi-column table with dozens of rows as well as with a single column table with only one result. The modular alert does not add the search results to the incident. Nor does it create multiple incidents when the result count is greater than one.
Is the documentation correct but the MA is not behaving as it should or is the documentation incorrect and we should be expected to use the Splunk Drilldown button from now on? The latter is a poor option as SIDs expire too quickly and Splunk alert search results should not be held as a system of record--that's the whole point of creating an incident.
I can add information to an incident with the | snowincident command's "--comments" option but I don't expect my Splunk users to know how to graft search results to a data generating command (that is a significant bit of acrobatics we shouldn't have to do based on the bahaviour expected from the modular alert action).