Remove outdated Belt.Array documentation regarding stdlib's Array.get

[mediremi]

Since v12 (or since v11 if using Core), the standard library's Array.get returns an option instead of throwing an exception if an index isn't found.

I've removed an outdated note from Belt_Array.resi that mentions the old behaviour of Array.get.

Also, since v11, arr[0] in ReScript is compiled to arr[0] in JavaScript, contrary to what the Belt_Array.resi docs said.

[pkg-pr-new]

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rescript

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@rescript/darwin-arm64

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@rescript/darwin-x64

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@rescript/linux-arm64

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@rescript/linux-x64

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@rescript/win32-x64

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commit: ee88861

[cknitt]

Thanks!