[Reporting/CSV/Docs] Specify maximum value for scroll.duration setting (

elastic#205765)

## Summary

Updates Reporting CSV export documentation to specify there is a maximum
value for `xpack.reporting.csv.scroll.size`, which is 10000.

(cherry picked from commit c3c6bfb)

docs/settings/reporting-settings.asciidoc

@@ -225,7 +225,7 @@ in time API], or {ref}/sql-rest-format.html#_csv[SQL with CSV response data form
 ============
 
 `xpack.reporting.csv.scroll.size`::
-Number of documents retrieved from {es} for each scroll iteration during a CSV export. Defaults to `500`.
+Number of documents retrieved from {es} for each scroll iteration during a CSV export. The maximum value is `10000`. Defaults to `500`.
 [NOTE]
 ============
 You may need to lower this setting if the default number of documents creates a strain on network resources.