[2.0.21] Clarify the relationship between recipe allow_productivity and item weight

[warbob5000]

As per the related not-a-bug topic, item weight is affected by a recipe's allow_productivity flag. Although it is stated that the weight is calculated automatically from recipes, it is unintuitive that allow_productivity=false adjusts the weight such that rocket capacity is at most 1 stack (rocket capacity for allow_productivity=true being rounded down to the nearest stack size is also undocumented, but is at least intuitive).

I suggest including that information, possibly by expanding the item.weight description, or perhaps by going into more detail over said automatic weight calculation (i.e. is allow_productivity the only flag that affects it? What happens if there are multiple recipes for the same item?)