Clarify configure SAST with customizations docs

What does this MR do and why?

While investigating https://gitlab.com/gitlab-com/sec-sub-department/section-sec-request-for-help/-/issues/357+s, I consulted the Configure SAST with customizations docs and noticed that they're incorrect.

The docs currently explain:

1. If the project does not have a `.gitlab-ci.yml` file, select **Enable SAST** in the Static
   Application Security Testing (SAST) row, otherwise select **Configure SAST**.

These docs indicate that if a .gitlab-ci.yml file is present, the button will show Configure SAST, however, this is not the case. In order for the button to show Configure SAST "[the latest] pipeline must have been successfully executed and generated valid artifacts" for the SAST scanner to be considered enabled, as explained in Fix text about Ultimate features and clarify co... (!100015 - merged) • Sam White • 15.5.

This MR clarifies the condition required for the button to show Enable SAST or Configure SAST.

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