[DOCS] - Improve image pull secrets documentation clarity

What does this MR do?

I recently ran across this issue comment from 2 years ago gitlab#20467 (comment 215095526) because I was having trouble getting the gitlab helm chart to properly configure the kubernetes executor to use a private docker registry. Once I found the above comment I was able to configure the helm chart properly.

It would be nice if the official documentation was clearer on these values/parameters so that configuring the kubernetes executor with private registry credentials is a more obvious procedure in the future.

This MR attempts to clarify the different options for configuring the kubernetes executor.

Related issues

gitlab#20467 (comment 215095526)

Author's checklist

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