Warn against custom request factories in RestTemplateBuilder

This commit adds a javadoc note about a usability issue described in
gh-11255. While `RestTemplateBuilder` is an immutable class, providing a
custom instance of request factory and deriving several
builders/templates from that point may have some unexpected behavior,
since that instance is shared amongst builders instances.

This issue is fixed in Spring Boot 2.0 with a replacement method that
leverages a `Supplier<ClientHttpRequestFactory>` instead.

See gh-11255

spring-boot/src/main/java/org/springframework/boot/web/client/RestTemplateBuilder.java

@@ -337,6 +337,8 @@ public class RestTemplateBuilder {
 	/**
 	 * Set the {@link ClientHttpRequestFactory} that should be used with the
 	 * {@link RestTemplate}.
+   * <p>Note that this request factory will be shared with all builder instances
+   * derived from that point.
 	 * @param requestFactory the request factory to use
 	 * @return a new builder instance
 	 */