Docs: Edited for style and to clarify how the variables work

What does this MR do?

As I was reading this topic, I came across a statement I didn't understand:

The `MYSQL_DATABASE` and `MYSQL_ROOT_PASSWORD` variables can't be set in the GitLab UI.
To set them, assign them to a variable [in the UI](../variables/README.md#via-the-ui), 
and then assign that variable to the 
`MYSQL_DATABASE` and `MYSQL_ROOT_PASSWORD` variables in your `.gitlab-ci.yml`.

I dug into the issue with @marcel.amirault and tested in a test project to confirm the functionality, then re-wrote that bit and updated the example to be more clear.

While I was looking at the topic, I edited it for style and clarity.

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