docs: update data-sveltekit-keepfocus docs

[geoffrich]

Tweak the wording and add additional information around data-sveltekit-keepfocus

• add note at the top about these options also applying to forms, so that it's not surprising when we use a form for one of the options when the section is called "link options"
• link existing focus reset docs
• remove "focus will be reset to the body" -- I think it's better just to link the existing focus reset docs
• more context on why using this on links or elements that don't exist on the new page is bad practice

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[dummdidumm]

I think this adds some good points but also removes the info that using this on links will in fact make the a tag focussed - users may think focus will stay on the previous element. Can we add that back?