When using Testcontainers at development time with @ServiceConnection on a @Bean method, describe when to use the name attribute and why

[wilkinsona]

See #36064 and in particular #36064 (comment) for background. When using Testcontainers at development time with @ServiceConnection on a @Bean method, there's an increased need to use the name attribute of @ServiceConnection as the container instance isn't available for us to call getDockerImageName() on it. We should document this, describing when to use the name attribute and explaining why it's necessary.