Decoration: Add support for zero probability, fix breakage from last commit

doc/lua_api.txt

@@ -432,14 +432,22 @@ Schematic specifier
 or through raw data supplied through Lua, in the form of a table.  This table must specify two fields:
  - The 'size' field is a 3d vector containing the dimensions of the provided schematic.
  - The 'data' field is a flat table of MapNodes making up the schematic, in the order of [z [y [x]]].
+
 In the bulk MapNode data, param1, instead of the typical light values, instead represents the 
-probability of that node appearing in the structure.  It is an integer with a value from 0-255; 0 means
-that node will always appear.  If the probability value p is non-zero, then there is a
-(p / 256 * 100)% chance that node will appear when the schematic is placed on the map.
+probability of that node appearing in the structure.
+When passed to minetest.create_schematic, probability is an integer value ranging from -1 to 255:
+ - A probability value of 0 means that node will always appear.
+ - A probability value of -1 means the node will never appear.
+ - If the probability value p is greater than 0, then there is a (p / 256 * 100)% chance that node
+   will appear when the schematic is placed on the map.
+
+If registering a structure in the raw format, however, -1 is not a valid probability value; in order to
+have a node that is not placed, it must be CONTENT_IGNORE (the name for which is "ignore").
+
 Important note: Node aliases cannot be used for a raw schematic provided when registering as a decoration.
 
 Schematic attributes
------------------
+---------------------
 Currently supported flags:  place_center_x, place_center_y, place_center_z
  - place_center_x
     Placement of this decoration is centered along the X axis.
@@ -1281,7 +1289,7 @@ minetest.create_schematic(p1, p2, probability_list, filename)
 ^ Apply the specified probability values to the specified nodes in probability_list.
    ^ probability_list is an array of tables containing two fields, pos and prob.
    ^ pos is the 3d vector specifying the absolute coordinates of the node being modified,
-   ^ and prob is the integer value from 0 to 255 of the probability (see: Schematic specifier).
+   ^ and prob is the integer value from -1 to 255 of the probability (see: Schematic specifier).
    ^ If there are two or more entries with the same pos value, the last occuring in the array is used.
    ^ If pos is not inside the box formed by p1 and p2, it is ignored.
    ^ If probability_list is nil, no probabilities are applied.