Mention "flip" in LuaEntity.mirroring

[sparr]

The event and hotkey names are based on "flip" and "flipped", so it's non-obvious to find the "mirroring" property if you don't have that search term. It would be helpful if the description of LuaEntity.mirroring included the word "flip" or "flipped". And/or if the description of on_entity_flipped mentioned mirroring.

[Therenas]

Yeah that's a good point, I added references to both sides of this for the next release, thanks.