Add comments in scripts

What does this MR do?

During understanding what start and related scripts in /scripts do, I added some comments for not widely known shell commands and replaced '.' by source. Now it should be a bit quicker for newcomers to understand the first steps after cloning the repo.

How confident are you it won't break things if deployed?

Very sure: scripts/start and scripts/test run as expected: only the test ChatCest fails and it fails on master as well (time out of 10 seconds to receive message, maybe fails due to slow computer setup, test works in gitlab-pipeline). No testing actually necessary since changes are only touching code style, not content.