doc: Update load-testing documentation with WebSocket note (#5778) (#5782)

Added note about recording proxy limitations with WebSockets.

Co-authored-by: Tatu Lund <tatu@vaadin.com>

Author: vaadin-bot

articles/flow/testing/load-testing/index.adoc

@@ -121,6 +121,9 @@ When it is not -- for example, during normal IDE runs -- the helper is a no-op a
 
 The test body itself is just a regular TestBench test -- see <<{articles}/flow/testing/end-to-end/creating-tests#, Creating Tests>> for the element query and interaction APIs.
 
+[NOTE]
+At the moment the recording proxy does not support WebSockets. Thus you need to configure the application under test to use LONG_POLLING when being tested. 
+
 [[destructive]]
 === Excluding Destructive Tests
 
@@ -827,4 +830,4 @@ mvn loadtest:run \
 - *Scale gradually.* Start with a small number of virtual users and increase to find breaking points.
 - *Use a stable network.* For cloud testing, run the load generator in the same region as the application server.
 - *Pre-record tests.* Record on a development machine, then distribute the generated k6 scripts to the load generation environment.
-- *Monitor the server.* Enable Actuator metrics collection (`k6.collectVaadinMetrics=true`) to correlate server health with k6 results.
+- *Monitor the server.* Enable Actuator metrics collection (`k6.collectVaadinMetrics=true`) to correlate server health with k6 results.