Remove mentions to 640x400 x 320x200 from Plugin docs

[ericoporto]

So, to quickly write the plugin documentation when porting the plugin information that was in the website I copy pasted the information from the website.

Turns out that there's non-updated things mentioned there. One of these, is the mention to 640x400 and 320x200 old quirky coordinate system. These observations are incorrect for how AGS works today, so we need to update this in the docs

https://github.com/adventuregamestudio/ags-manual/wiki/EnginePluginRun-timeAPI

[ivan-mogilko]

I guess this kind of obsolete info may be found elsewhere, so scanning all the manual would be needed.

This is similar to #161.

For the reference, quoting a quick explanation:

Back in a day AGS did not use proper graphics scaling to the window size, and instead scaled whole game coordinate system. So, running 320x200 game scaled x2 would actually scale not only the gfx, but also the inner logic.

Today all this is in past, and game's logic always works with the native coordinates.