Add a comment with an example of filter push down

Author: afoucret

x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/rules/logical/PushDownAndCombineFiltersTests.java

@@ -245,6 +245,9 @@ public void testSelectivelyPushDownFilterPastFunctionAgg() {
         assertEquals(expected, new PushDownAndCombineFilters().apply(fb));
     }
 
+
+    // from ... | where a > 1 | COMPLETION "some prompt" WITH reranker AS completion | where b < 2 and match(completion, some text)
+    // => ... | where a > 1 AND b < 2| COMPLETION "some prompt" WITH reranker AS completion | match(completion, some text)
     public void testPushDownFilterPastCompletion() {
         FieldAttribute a = getFieldAttribute("a");
         FieldAttribute b = getFieldAttribute("b");