Improve documentation of FIPS-enabled images in SAST

What does this MR do and why?

This merge request updates the FIPS-enabled images section in SAST documentation to clarify the need to use the correct user ID when running FIPS-enabled Semgrep image in a kubernetes-based runner with a non-root user.

This is based on a discussion with one of the support engineers where a customer was having an issue running Semgrep in a non-root based runner.

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