specs/weaschem: clarify about node names

.docs/specs/WorldEditAdditionsSchematic_v1.md

@@ -247,6 +247,7 @@ Delta schematic files, as described above, store only the changes made to a give
 
 See also the specific notes in the sections below.
 
+
 ## ID map
 Like the [header](#header), the ID map is also defined as a JSON object on a single line, followed by a new line character (`\n` U+0A):
 
@@ -262,6 +263,8 @@ The ID map's purpose is to map node IDs in the schematic file to canonical node
 
 Node names in a schematic file MUST be the full canonical node name, and not an abbreviation. For example, `default:stone` is valid, but `stone` is invalid (does not specify the mod).
 
+Node IDs in the id map MUST be an integer, and MUST NOT be less than 0. Specifically, node ids in the id map that are less than 0 are considered invalid, and node ids greater than -1 are considered valid. See also [special node ids](#special-node-ids).
+
 Implementers MAY perform arbitrary transformations on node names when reading or writing a WorldEditAdditions schematic file, but any such transformations are beyond the scope of this specification.
 
 When loading a schematic, it is RECOMMENDED that implementers use a cache when translating schematic node ids (from this ID map) to node ids from the target world.
@@ -350,4 +353,3 @@ Node ID	| Purpose
 --------|-----------------
 -1		| Represents a value of null/nil. This node id value indicates that no node is stored in this cell. When reading a schematic and writing it to the world, implementers MUST NOT alter the pre-existing node in the world that corresponds with this cell.
 -2		| **In a delta schematic only,** This node id value indicates that no change was made between the time the schematic was written and the previous time it was compared against. Using this value as a node in a schematic with a `type` of `full` is **invalid**, and in such a case implementers MUST raise an error message and abort reading of the schematic.
-