`/forks` should document the github apps limitations

[xmo-odoo]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/rest/repos/forks#create-a-fork

What part(s) of the article would you like to see updated?

POST /repos/{owner}/{repo}/forks is currently documented as

This endpoint works with the following fine-grained token types:

GitHub App user access tokens
GitHub App installation access tokens
Fine-grained personal access tokens

There is no hint or indication that none of these work when forking between private repositories of different organizations.

Furthermore the feedback received when attempting this operation using GHA tokens are of an extreme level of unhelpfulness: given a GHA installed in both organizations A and B as well as a user account which has admin access to both, trying to fork a private repository A.r into B

• Using the installation access token (IAT) for A, the fork returns a success, but the organization field is ignored and it creates the fork into A.
• Using the IAT for B, the fork fails with a 404 (this makes some sense, although the feedback is unhelpful).
• Using the user access token for the user, the fork returns a 403 with the error message "resource not accessible by integration".

Additional information

• REST Forks apis that are not actually github apps friendly #21806 is an older version of this report
• GitHub App can't fork repositories community/community#24607 is the earlier report on the subject