docs: nested unpacking in for is not allowed

[tlienart]

{{for (i, (a, b)) in enumerate(iter)}}
...
{{end}}

not supported, will have to do something like {{for (i, e) in enumerate(iter)}} {{> $e[1]}} ... (or shove that in utils)

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may be possible to do in the future but difficulty is to keep track of the variable names at arbitrary depth, seems easier to just not support this for now.