Spring-ai Add comment in OpenAiImageModel and modify RetryTemplate access level in OpenAiChatModel

• add enough comment in OpenAiImageModel like OpenAiChatModel format.
• changed public to private in OpenAiChatModel RetryTemplate.

resolved #755

Comment From: markpollack

thanks, merged in 0e0d453e2bea9ddca062b6e32435944833847589