site/docs improvements

docs/when.md

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 # The `when` statement (expression)
 
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 Lyng provides a concise multi-branch selection with `when`, heavily inspired by Kotlin. In Lyng, `when` is an expression: it evaluates to a value. If the selected branch contains no value (e.g., it ends with `void` or calls a void function like `println`), the whole `when` expression evaluates to `void`.
 
 Currently, Lyng implements the "subject" form: `when(value) { ... }`. The subject-less form `when { condition -> ... }` is not implemented yet.