[python-package] [docs] Update key format of eval_hist in docstring example

[Alnusjaponica]

Although the key format of eval_hist, which is return value of lightgbm.cv, is changed by https://github.com/microsoft/LightGBM/pull/2089, the document is not properly updated. This PR updates docstrings of lightgbm.cv.

[Alnusjaponica]

@jameslamb Thanks for the comment. I updated the name as you suggested. I also updated the description to make it more detailed. Please let me know if it is not necessary.

[Alnusjaponica]

Since eval_train_metric is not propagated to _agg_cv_result, it seems eval_results keys always have prefix "valid " or "train ". I'll confirm if it is true and update this PR.

[Alnusjaponica]

Hi, I updated the PR but cuda 11.7.1 wheel (linux, gcc, Python 3.11) seems to have failed unexpectedly. Could you re-run the CI?

[neverfox]

Isn't this prefix a breaking change? I just ran into it going from 3.3.4 to 4.1.0. Where could I find that in the release notes as such?

[jameslamb]

@neverfox please open a new issue referencing this, and we. can discuss there. Let's not continue this discussion in the comments of an already-closed PR please.

[github-actions[bot]]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.