Document how "preferred_environment" and "scripts" interact in Safari

[dotproto]

In Safari (18+) only, if preferred_environment is set to service_worker AND background.service_worker is absent AND background.scripts is present, then these scripts are all executed in a service worker context.

Originally posted by @Rob--W in #38635 (comment)

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• Update the Safari section of the Browser support section.
• The last paragraph in the "preferred_environment" description needs to be clarified.

CC: @xeenon, @Rob--W, @rebloor

[dotproto]

This is out of scope, but thinking out loud for a moment, I wonder if it would be better to describe the sub-properties in sections rather than in a table. While reading the "preferred_environment" row, I found myself thinking "it would be nice to see an examples of what a background definition using this property looks like and a summary of how it would behave in each of the 3 browser engines."

Now that I've written the thought it, it feels like it might be a bit much to try to include that here. Perhaps we could cover some of that in an examples section? A blog post?