Add note on scope for internal projects

What does this MR do?

Currently, project access tokens are able to access Internal projects if it is created by an internal user as stated in the warning message for Creating project token which can be easily missed when users are reading the docs. If we look at the Scopes for Project Access Token section, it does not mention anything about internal projects, which implies that the access token is only limited to whichever project it is created from. This may be setting the wrong expectation as users were confused that their project access token was able to access another project

curl --request GET --header "PRIVATE-TOKEN: <private-token>" "https://gitlab.example.com/api/v4/projects"

Related issues

This issue came up in this internal ticket

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