add item.caption(...) function & move item library docs to libitem.md

doc/en/scripting/builtins/libblock.md

@@ -10,7 +10,7 @@ block.index(name: str) -> int
 -- Returns the id of the block material.
 block.material(blockid: int) -> str
 
--- Returns the block name displayed in the UI.
+-- Returns the block display name.
 block.caption(blockid: int) -> str
 
 -- Returns integer ID by block position

doc/en/scripting/builtins/libitem.md

@@ -0,0 +1,21 @@
+# *item* library
+
+```lua
+-- Returns item string ID (name) by index
+item.name(itemid: int) -> str
+
+-- Returns item integer ID (index) by name
+item.index(name: str) -> int
+
+-- Returns the item display name.
+block.caption(blockid: int) -> str
+
+-- Returns max stack size for the item
+item.stack_size(itemid: int) -> int
+
+-- Returns count of available item IDs.
+item.defs_count() -> int
+
+-- Returns item icon name to use in 'src' property of an image element
+item.icon(itemid: int) -> str
+```