Update confusing and obsolete sentence in Pipeline SD documentation

What does this MR do and why?

This merge request updates a sentence that is both confusing (internal, slack, only available for 90 days) and obsolete (was introduced while the fix in #425251 (closed) was still being implemented).

The updated sentence still highlights the fact you cannot combine synthesizing a custom configuration with a remote ruleset as this is still an existing limitation that is planned to be fixed with #336395 (closed).

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