Recover Deleted XNLI Instructions
# What does this PR do?

This reintroduces the instructions and task description of XNLI to the examples documentation for text classification. These seem to have been inadvertently deleted during the examples re-org since they followed the instructions for Tensorflow instructions for `run_glue.py`. You can see the deletion [here](https://github.com/huggingface/transformers/pull/11350/files#L206).

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## Before submitting
- [x] This PR fixes a typo or improves the docs.


## Who can review?

The reorg was done by @sgugger, so he can best confirm that the deletion of these was inadvertent.