Docs: Add search response took time explanation (#26202)

docs/reference/search/request-body.asciidoc

@@ -155,6 +155,13 @@ be set to `true` in the response.
 --------------------------------------------------
 // TESTRESPONSE[s/"took": 3/"took": $body.took/]
 
+The `took` time in the response contains the milliseconds that this request
+took for processing, beginning quickly after the node received the query, up
+until all search related work is done and before the above JSON is returned
+to the client. This means it includes the time spent waiting in thread pools,
+executing a distributed search across the whole cluster and gathering all the
+results.
+
 include::request/query.asciidoc[]
 
 include::request/from-size.asciidoc[]