📚 Add comment to workflow files

[ckunki]

Background & Context

@tkilias and I were wondering and discussing why the following section is included in the GitHub workflows proposed by PTB:

  pull_request:
    types: [opened, reopened]

We imagined the following rationale:

• At the time when the pr is opened or reopened, the check results of former push operation could already be outdated
• So the PR should run the checks again

Acceptance Criteria

• A comment is added to the workflow files provided by the PTB, e.g. # check results of former push could be outdated
• Optionally: An additional section is added to the developer guide explaining rationale and mitigation in more details.