docs: clarify docs for svelte/no-target-blank

[benmccann]

The behavior of allowReferrer was very confusing to me because I thought it was referring to the rel attribute and I did not understand it's actually referring to the Referer header being allowed

I was also very confused about the security vulnerability being referred to. I assumed it was related to protecting the user's privacy by not passing the referrer header. When I happened to click the reference link I saw that I had been misunderstanding the purpose of this rule and what exactly it was doing.

[changeset-bot]

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[ota-meshi]

Thank you!