docs: add migration information on how to use a different http client

[skip ci]

Author: metacosm

docs/documentation/v4-3-migration.md

@@ -9,16 +9,16 @@ permalink: /docs/v4-3-migration
 
 ## Condition API Change
 
-In Workflows the target of the condition was the managed resource itself, not the target dependent resource. 
+In Workflows the target of the condition was the managed resource itself, not the target dependent resource.
 This changed, now the API contains the dependent resource.
 
 New API:
 
 ```java
 public interface Condition<R, P extends HasMetadata> {
-    
-  boolean isMet(DependentResource<R, P> dependentResource, P primary, Context<P> context);
-  
+
+    boolean isMet(DependentResource<R, P> dependentResource, P primary, Context<P> context);
+
 }
 ```
 
@@ -27,10 +27,24 @@ Former API:
 ```java
 public interface Condition<R, P extends HasMetadata> {
 
-  boolean isMet(P primary, R secondary, Context<P> context);
-  
+    boolean isMet(P primary, R secondary, Context<P> context);
+
 }
 ```
 
-Migration is trivial. Since the secondary resource can be accessed from the dependent resource. So to access the secondary
+Migration is trivial. Since the secondary resource can be accessed from the dependent resource. So to access the
+secondary
 resource just use `dependentResource.getSecondaryResource(primary,context)`.
+
+## HTTP client choice
+
+It is now possible to change the HTTP client used by the Fabric8 client to communicate with the Kubernetes API server.
+By default, the SDK uses the historical default HTTP client which relies on Okhttp and there shouldn't be anything
+needed to keep using this implementation. The `tomcat-operator` sample has been migrated to use the Vert.X based
+implementation. You can see how to change the client by looking at
+that [sample POM file](https://github.com/java-operator-sdk/java-operator-sdk/blob/d259fcd084f7e22032dfd0df3c7e64fe68850c1b/sample-operators/tomcat-operator/pom.xml#L37-L50):
+
+- You need to exclude the default implementation (in this case okhttp) from the `operator-framework` dependency
+- You need to add the appropriate implementation dependency, `kubernetes-httpclient-vertx` in this case, HTTP client
+  implementations provided as part of the Fabric8 client all following the `kubernetes-httpclient-<implementation name>`
+  pattern for their artifact identifier.