Update reservable docs to reflect actual ed (#13894)

* Update reservable docs to reflect actual ed

Update reservable documentation to reflect the actual behavior of small reserved balances.

Looking into `frame/balances/src/impl_currency.rs` shows that accounts are not reaped even if the reserved balance is very small.

* ".git/.scripts/commands/fmt/fmt.sh"

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Co-authored-by: command-bot <>