Generate: better warnings with pipelines
# What does this PR do?

Addresses comments in https://github.com/huggingface/transformers/issues/23054

This PR adds the following enhancements to generate-related pipeline warnings:
1. Clarifies the `max_length` reduction suggestion in the summarization pipeline
2. Also pipes task-specific configuration to `generation_config` (when applicable), which fixes the warning about relying on `model.config` to parameterize `generate`
_The documentation is not available anymore as the PR was closed or merged._