feat: Add a note to the dataset and table descriptions

[ricalo]

The note explains that there is a delay from the time the release notes are published in the HTML versions until they are available in the google_cloud_release_notes dataset.

The note is required because there are user reports that indicate that users expect the release notes to be synchronized between the different locations. The note sets expectations accordingly.

Description

Notes:

• If you are adding or editing a dataset, please specify the dataset folder involved, e.g. datasets/google_trends.
  The dataset folder involved is datasets/google_cloud_release_notes.
• If you are an external contributor, please contact the Google Cloud Datasets team for your proposed dataset or feature.
  Not an external contributor. I'm part of the Google Cloud Platform organization on GitHub.
• If you are adding or editing a dataset, please do it one dataset at a time. Have all the code changes inside a single datasets/<DATASET_NAME> folder.
  This pull request only updates the Google Cloud Release Notes dataset in the datasets/google_cloud_release_notes folder.

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/

[ricalo]

It looks to me like the Lint Python check is failing for reasons unrelated to this PR. Sending this for review.