Fix generation docstrings regarding input_ids=None
# What does this PR do?

The docstrings incorrectly describe what happens if no input_ids are passed. This PR changes the docstrings so that the actual behavior is described.

## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

Documentation: @sgugger

Thanks for reviewing!
The merge conflict has been resolved.