Highlight why dotenv variables might not be available

What does this MR do?

This is to address what lead to the creation of #390115 (closed)

The conditions to make a job not receive dotenv variables are not necessarily very intuitive. In the section on how to use dotenv variables we only say "subsequent jobs get them", without any mention of possible limitations to that sentence. While those limitations are listed right below, the issue linked above is a good showcase that this can still be pretty easy to miss. So my idea would be to just say "there might be limitations to this!" right next to "subsequent jobs get them".

Related issues

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