Document transport unloading types

There's three unloading types you can choose via the transport's unit def `transportUnloadMethod` tag.

Document what they do:
* basic behaviour, perhaps with examples/analogies (tldr 0 is TA style, 1 is parachutes, 2 is every other RTS transport ever)
* what script callins are involved?
* what other unit def tags (incl. on transportees) are relevant?
* air vs ground transport?
* what's the behaviour when looking for an empty space? When space becomes occupied?
* what's the behaviour when generating commands (e.g. what if somebody hops on while an unload command was already given, is he unloaded)?
* any other details.

No file or test is named. Start by locating the transportUnloadMethod handling and related unload script callins, then document modes 0, 1, and 2, relevant transport and transportee tags, air versus ground behavior, empty-space occupancy, and command-generation cases. Done means the documentation covers each listed behavior with examples where useful.

Written by the indexing model from the issue text.