docs(http): heads up about provider ordering (angular#56518)

PR Close angular#56518

Author: CodeF0x

adev/src/content/guide/http/testing.md

@@ -8,7 +8,9 @@ At the end, tests can verify that the app made no unexpected requests.
 
 ## Setup for testing
 
-To begin testing usage of `HttpClient`, configure `TestBed` and include `provideHttpClient()` and `provideHttpClientTesting` in your test's setup. This configures `HttpClient` to use a test backend instead of the real network. It also provides `HttpTestingController`, which you'll use to interact with the test backend, set expectations about which requests have been made, and flush responses to those requests. `HttpTestingController` can be injected from `TestBed` once configured.
+To begin testing usage of `HttpClient`, configure `TestBed` and include `provideHttpClient()` and `provideHttpClientTesting()` in your test's setup. This configures `HttpClient` to use a test backend instead of the real network. It also provides `HttpTestingController`, which you'll use to interact with the test backend, set expectations about which requests have been made, and flush responses to those requests. `HttpTestingController` can be injected from `TestBed` once configured.
+
+Keep in mind to provide `provideHttpClient()` **before** `provideHttpClientTesting()`, as `provideHttpClientTesting()` will overwrite parts of `provideHttpCient()`. Doing it the other way around can potentially break your tests.
 
 <docs-code language="ts">
 TestBed.configureTestingModule({