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