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