Main menu, lua_api.txt: Fix mod/texture pack screenshot size issues

Recommend mod screenshot size in lua_api.txt
Adjust displayed screenshot size of texture packs
Document texture pack files in lua_api.txt

builtin/mainmenu/tab_texturepacks.lua

@@ -96,8 +96,8 @@ local function get_formspec(tabview, name, tabdata)
	return	retval ..
			render_texture_pack_list(list) ..
			";" .. index .. "]" ..
			"image[0.25,0.25;4.0,3.7;" .. core.formspec_escape(screenfile or no_screenshot) .. "]" ..
			"textarea[0.6,3.5;3.7,1.5;;" .. core.formspec_escape(infotext or "") .. ";]"
			"image[0.25,0.25;4.05,2.7;" .. core.formspec_escape(screenfile or no_screenshot) .. "]" ..
			"textarea[0.6,2.85;3.7,1.5;;" .. core.formspec_escape(infotext or "") .. ";]"
end

--------------------------------------------------------------------------------

doc/lua_api.txt

@@ -157,7 +157,8 @@ to a single modname. Their meaning is that if the specified mod
is missing, that does not prevent this mod from being loaded.

### `screenshot.png`
A screenshot shown in modmanager within mainmenu.
A screenshot shown in the mod manager within the main menu. It should
have an aspect ratio of 3:2 and a minimum size of 300×200 pixels.

### `description.txt`
A File containing description to be shown within mainmenu.
@@ -234,6 +235,40 @@ stripping out the file extension:
* e.g. `foomod_foothing.png`
* e.g. `foomod_foothing`

Texture pack directory structure
--------------------------------

    textures
    |-- Texture Pack
    |   |-- screenshot.png
    |   |-- description.txt
    |   |-- your_texture_1.png
    |   |-- your_texture_2.png
    `-- Another Texture Pack

### Texture Pack
This is a directory containing the entire contents of a single texture pack.
It can be chosen more or less freely and will also become the name of the
texture pack. The name must not be “base”.

### `description.txt`
A file containing a short description of the texture pack to be shown in the
texture packs tab.

### `screenshot.png`
A preview image showing an in-game screenshot of this texture pack; it will be
shown in the texture packs tab. It should have an aspect ratio of 3:2 and a
minimum size of 300×200 pixels.

### `your_texture_1.png`, `your_texture_2.png`, etc.
Any other PNG files will be interpreted as textures. They must have the same
names as the textures they are supposed to override. For example, to override
the apple texture of Minetest Game, add a PNG file named `default_apple.png`.

The custom textures do not necceessarily require the same size as their
originals, but this might be required for a few particular textures. When
unsure, just test your texture pack in-game.

Texture modifiers
-----------------
There are various texture modifiers that can be used