[`BLIP`] fix doctest
# What does this PR do?

This PR fixes `BLIP` doctest. 

Link to failing job: https://github.com/huggingface/transformers/actions/runs/3964164193 

The docstring of the `forward` method of `BlipForQuestionAnswering` has been corrected to educate users on how to correctly use this module after https://github.com/huggingface/transformers/pull/21021 being merged. 
The logic now of the `forward` method is pretty much similar to [`T5`](https://github.com/huggingface/transformers/blob/main/src/transformers/models/t5/modeling_t5.py#L1592-L1610).  

cc @ydshieh 💯 
_The documentation is not available anymore as the PR was closed or merged._