doc: Replace all (allow-none) annotations by (optional) and/or (nullable)
The (allow-none) annotation is deprecated since a long time now, it is better to use (nullable) and/or (optional) which clarifies what it means with the (out) annotation.
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The (allow-none) annotation is deprecated since a long time now, it is better to use (nullable) and/or (optional) which clarifies what it means with the (out) annotation.