Site docs: overloads could be more visible

* `SetUnitMass` is a function with one variant, it says `(function)`.
* `SetUnitPosition` has two variants, one says `(function)` and the other says `(function overload 1)`. The link to the overload is `https://recoilengine.org/docs/lua-api/#Spring.SetUnitPosition-1`.
* this makes it hard to realize there's an overload since the "base" variant looks like any other function and nothing hints that there is an overload. It would be nice if the variants were called "function overload 1" and "function overload 2" with neither being designated as a "base" and called just "function". Perhaps even better if a "function" section did exist and explicitly stated that there are two overloads.

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Start with the generated Lua API documentation at docs/lua-api/, comparing SetUnitMass with SetUnitPosition and its overload link at #Spring.SetUnitPosition-1. Update the presentation so overloaded variants are visibly grouped or consistently labeled, then verify that single functions remain clear and both SetUnitPosition variants are discoverable.

Written by the indexing model from the issue text.