Update docs on testing client code

Closes gh-34892

Author: rstoyanchev

framework-docs/modules/ROOT/pages/testing/spring-mvc-test-client.adoc

@@ -1,10 +1,26 @@
 [[spring-mvc-test-client]]
 = Testing Client Applications
 
-You can use client-side tests to test code that internally uses the `RestTemplate`. The
-idea is to declare expected requests and to provide "`stub`" responses so that you can
-focus on testing the code in isolation (that is, without running a server). The following
-example shows how to do so:
+To test code that uses the `RestClient` or `RestTemplate`, you can use a mock web server, such as
+https://github.com/square/okhttp#mockwebserver[OkHttp MockWebServer] or
+https://wiremock.org/[WireMock]. Mock web servers accept requests over HTTP like a regular
+server, and that means you can test with the same HTTP client that is also configured in
+the same way as in production, which is important because there are often subtle
+differences in the way different clients handle network I/O. Another advantage of mock
+web servers is the ability to simulate specific network issues and conditions at the
+transport level, in combination with the client used in production.
+
+In addition to dedicated mock web servers, historically the Spring Framework has provided
+a built-in option to test `RestClient` or `RestTemplate` through `MockRestServiceServer`.
+This relies on configuring the client under test with a custom `ClientHttpRequestFactory`
+backed by the mock server that is in turn set up to expect requests and send "`stub`"
+responses so that you can focus on testing the code in isolation, without running a server.
+
+TIP: `MockRestServiceServer` predates the existence of mock web servers. At present, we
+recommend using mock web servers for more complete testing of the transport layer and
+network conditions.
+
+The following example shows an example of using `MockRestServiceServer`:
 
 [tabs]
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framework-docs/modules/ROOT/pages/web/webflux-webclient/client-testing.adoc

@@ -2,9 +2,16 @@
 = Testing
 :page-section-summary-toc: 1
 
-To test code that uses the `WebClient`, you can use a mock web server, such as the
-https://github.com/square/okhttp#mockwebserver[OkHttp MockWebServer]. To see an example
-of its use, check out
+To test code that uses the `WebClient`, you can use a mock web server, such as
+https://github.com/square/okhttp#mockwebserver[OkHttp MockWebServer] or
+https://wiremock.org/[WireMock]. Mock web servers accept requests over HTTP like a regular
+server, and that means you can test with the same HTTP client that is also configured in
+the same way as in production, which is important because there are often subtle
+differences in the way different clients handle network I/O. Another advantage of mock
+web servers is the ability to simulate specific network issues and conditions at the
+transport level, in combination with the client used in production.
+
+For example use of MockWebServer, see
 {spring-framework-code}/spring-webflux/src/test/java/org/springframework/web/reactive/function/client/WebClientIntegrationTests.java[`WebClientIntegrationTests`]
 in the Spring Framework test suite or the
 https://github.com/square/okhttp/tree/master/samples/static-server[`static-server`]