docs: fix navigation type JSDoc formatting

[vamshikrishnaramasamy]

Closes #15910.\n\nRemoves the repeated The type of navigation: prefix from each navigation type JSDoc item so editors don't merge the prose into every union member hover.\n\nChecks:\n- pnpm --dir packages/kit generate:types\n- pnpm --dir packages/kit lint

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[Rich-Harris]

This should have had a changeset since it affects the published package!

It also makes the docs look kinda goofy IMHO:

It would be better to put the descriptions on the interfaces themselves I think. Opened #16147

[teemingc]

This should have had a changeset since it affects the published package!
Whoops that's my bad.

It would be better to put the descriptions on the interfaces themselves I think. Opened #16147

Thanks! The docs look a lot better. For future context, a very minor thing that's lost from this is the description only showing the text from each type that's currently in the union. e.g., if the only possible type is popstate, it would only show the popstate description because it doesn't merge the descriptions from the types in the union.