Add warning to API about memory functions + Fix height based errors

[LuantistDork]

Add warning to API about memory functions:
(#6) Memory functions rely on staticdata, which means memory only saves when the object is unloaded. If the server crashes, then memory will be forgotten.

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Fix height based errors:
mobkit.hq_attack used tgtobj:get_luaentity().height, however only mobkit mobs store their height in their properties. I made it instead depend on the already defined mobkit.get_box_height