Document air_equivalent as deprecated

[sfan5]

No description provided.

[erlehmann]

If this is really deprecated, IMO the engine should warn in log every time this is accessed. After all, if you actually remove this property, something truthy may become falsy (nil).

”Unclear meaning” is not exactly correct, see: #13722 (comment)

[erlehmann]

Saying that it has an unclear meaning is still wrong, as discussed on IRC after I stopped a previous PR: #13790 (comment)

@SmallJoker what exactly changed since this was last discussion so that you approved it? What should people use instead of using air_equivalent? Both “is this air or is this ignore” and “does this have an airlike drawtype” are most certainly not the correct replacement. I ask this for the 32 mods on ContentDB which use air_equivalent: https://web.archive.org/web/20230911164208if_/https://content.minetest.net/zipgrep/501df1d6-4355-4fd8-8afb-947ea023aa6e/

[SmallJoker]

@erlehmann Node groups can be used to reflect such properties. Whether or not a node is air-like also depends on the interpretation. Air tanks might only consider breathable air variations as air-like (e.g. recharging air bottles), whereas machines might only care whether the node is some kind of non-explosive gas.
Thus it is best for mods to determine node groups or node definition checks that make sense for them to ensure the best possible compatibility.

PS: In my opinion it would've made sense to remove this undocumented field entirely. It's the modders fault to rely on undefined behaviour - much like UB in C++.