texture_packs.txt: Document special textures

doc/texture_packs.txt

@@ -44,6 +44,87 @@ Texture modifiers
 
 See lua_api.txt for texture modifiers
 
+Special textures
+----------------
+
+These texture names are hardcoded into the engine but can also be overwritten
+by texture packs. All existing fallback textures can be found in the directory
+`textures/base/pack`.
+
+### Gameplay textures
+
+* `bubble.png`: the bubble texture when the player is drowning
+
+* `crack_anylength.png`: node overlay texture when digging
+
+* `crosshair.png`
+    * the crosshair texture in the center of the screen. The settings
+      `crosshair_color` and `crosshair_alpha` are used to create a cross
+      when no texture was found
+
+* `halo.png`: used for the node highlighting mesh
+
+* `heart.png`: used to display the health points of the player
+
+* `minimap_mask_round.png`: round minimap mask, white gets replaced by the map
+* `minimap_mask_square.png`: mask used for the square minimap
+* `minimap_overlay_round.png`: overlay texture for the round minimap
+* `minimap_overlay_square.png`: overlay texture for the square minimap
+* `object_marker_red.png`: texture for players on the minimap
+* `player_marker.png`: texture for the own player on the square minimap
+
+* `player.png`: front texture of the 2D upright sprite player
+* `player_back.png`: back texture of the 2D upright sprite player
+
+* `moon.png`: texture of the moon. Default texture is generated by Minetest
+* `moon_tonemap.png`: tonemap to be used when `moon.png` was found
+* `sun.png`: texture of the sun. Default texture is generated by Minetest
+* `sun_tonemap.png`: tonemap to be used when `sun.png` was found
+* `sunrisebg.png`: shown sky texture when the sun rises
+
+* `smoke_puff.png`: texture used when an object died by punching
+
+* `unknown_item.png`: shown texture when an item definition was not found
+* `unknown_node.png`: shown texture when a node definition was not found
+* `unknown_object.png`: shown texture when an entity definition was not found
+
+* `wieldhand.png`: texture of the wieldhand
+
+### Mainmenu textures
+
+* `menu_bg.png`: used as mainmenu background when the clouds are disabled
+* `menu_header.png`: header texture when no texture pack is selected
+
+* `no_screenshot.png`
+    * texture when no screenshot was found for a texture pack or mod
+
+* `server_flags_creative.png`: icon for creative servers
+* `server_flags_damage.png`: icon for enabled damage on servers
+* `server_flags_favorite.png`: icon for your favorite servers
+* `server_flags_pvp.png`: icon for enabled PvP on servers
+
+### Android textures
+
+* `down_arrow.png`
+* `left_arrow.png`
+* `right_arrow.png`
+* `up_arrow.png`
+
+* `drop_btn.png`
+* `fast_btn.png`
+* `fly_btn.png`
+* `jump_btn.png`
+* `noclip_btn.png`
+
+* `camera_btn.png`
+* `chat_btn.png`
+* `inventory_btn.png`
+* `rangeview_btn.png`
+
+* `debug_btn.png`
+* `gear_icon.png`
+* `rare_controls.png`
+
 Texture Overrides
 -----------------