Document how to make nametags background disappear on players' head (m…

…inetest#10783)

Co-authored-by: Zughy <[email protected]>

doc/lua_api.txt

@@ -6938,7 +6938,11 @@ Player properties need to be saved manually.
  -- in mods.
 
  nametag = "",
- -- By default empty, for players their name is shown if empty
+ -- The name to display on the head of the object. By default empty.
+ -- If the object is a player, a nil or empty nametag is replaced by the player's name.
+ -- For all other objects, a nil or empty string removes the nametag.
+ -- To hide a nametag, set its color alpha to zero. That will disable it entirely.
+
 
  nametag_color = <ColorSpec>,
  -- Sets color of nametag