document the search context is freed if the scroll is not extended
The `fetchPhaseShouldFreeContext` returns true when there is a scroll context but the scroll parameter is null, thus freeing the search context.
@@ -109,7 +109,9 @@ request) tells Elasticsearch how long it should keep the search context alive.
Its value (e.g. `1m`, see <<time-units>>) does not need to be long enough to
process all data -- it just needs to be long enough to process the previous
batch of results. Each `scroll` request (with the `scroll` parameter) sets a
-new expiry time.
+new expiry time. If a `scroll` request doesn't pass in the `scroll`
+parameter, then the search context will be freed as part of _that_ `scroll`
+request.
Normally, the background merge process optimizes the
index by merging together smaller segments to create new bigger segments, at
