Clarify instructions in the docs for adding excluded paths

What does this MR do?

For one of the given scenarios as part of the Category Maturity Scorecard testing, we had a customer go to the docs and copy/paste the SAST_EXCLUDED_PATHS variable into their gitlab-ci.yml template (after which they added a docs path to exclude). In the process, he overrode the defaults (spec, test, tests, tmp). These defaults are listed in the docs, but I think we can do a better job of explaining that including this variable in the yml will override those defaults, and that if they want to add more paths, they'll have to , in addition, copy/paste the default paths as well.

See Slack thread (internal)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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cc @rdickenson @tmccaslin @twoodham @theoretick @rossfuhrman @ssarka @nmccorrison @markrian @jannik_lehmann @zrice