Update known problems for default_numeric_fallback

[Noratrieb]

The existing known problem is not actually true, and the many false positives and false negatives should be mentioned.

Allowing the lint on a single statement: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=328de43690af13ed126f7c00c15cd695

changelog: [default_numeric_fallback]: update known problems docs

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[Centri3]

I presume this was added because it used to not use span_lint_hir_and_then. Thanks, good catch!