Make PIT validation error actionable (#74224)

Closes #74223

Author: spinscale

server/src/main/java/org/elasticsearch/rest/action/search/RestSearchAction.java

@@ -315,7 +315,8 @@ static void preparePointInTime(SearchRequest request, RestRequest restRequest, N
         assert request.pointInTimeBuilder() != null;
         ActionRequestValidationException validationException = null;
         if (request.indices().length > 0) {
-            validationException = addValidationError("[indices] cannot be used with point in time", validationException);
+            validationException = addValidationError("[indices] cannot be used with point in time. Do " + 
+                "not specify any index with point in time.", validationException);
         }
         if (request.indicesOptions().equals(DEFAULT_INDICES_OPTIONS) == false) {
             validationException = addValidationError("[indicesOptions] cannot be used with point in time", validationException);

x-pack/plugin/async-search/qa/security/src/javaRestTest/java/org/elasticsearch/xpack/search/AsyncSearchSecurityIT.java

@@ -224,7 +224,7 @@ public void testRejectPointInTimeWithIndices() throws Exception {
             request.setJsonEntity(Strings.toString(requestBody));
             final ResponseException exc = expectThrows(ResponseException.class, () -> client().performRequest(request));
             assertThat(exc.getResponse().getStatusLine().getStatusCode(), equalTo(400));
-            assertThat(exc.getMessage(), containsString("[indices] cannot be used with point in time"));
+            assertThat(exc.getMessage(), containsString("[indices] cannot be used with point in time. Do not specify any index with point in time."));
         } finally {
             closePointInTime(pitId, authorizedUser);
         }