[`generate`]  Only warn users if the `generation_config`'s `max_length` is set to the default value
# What does this PR do?

Updates the condition for raising a warning in generate to warn users if the `generation_config`'s `max_length` is set to the default value. 
_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger as discussed on Slack with @ArthurZucker: `max_new_tokens` can't be used to set a default `generation_config` that works well out of the box -- `max_length` does. As such, let's adapt the warning to enable us (and the users) to set good default arguments.