Spring AI Update chatclient.adoc for call() return values

1. Need to add a description of the responseEntity() method within the call() return values section.
2. Need to add a Note in the documentation to clarify that calling the call() method does not actually trigger the AI model. It only tells SpringAI whether to use synchronous or streaming calls. The actual model call happens when invoke content(), chatResponse(), and responseEntity() (Or similar explanations).

Regarding point 2, since there are already two related issues (#3895 and #3880), it would be helpful to add a note in the documentation to explain this.