Schematic: Properly deal with before/after node resolving and document (#11011)

This fixes an out-of-bounds index access when the node resolver was already applied to the schematic (i.e. biome decoration).
Also improves the handling of the two cases: prior node resolving (m_nodenames), and after node resolving (manual lookup)

src/unittest/test_noderesolver.cpp

@@ -56,6 +56,8 @@ void TestNodeResolver::runTests(IGameDef *gamedef)
 
 class Foobar : public NodeResolver {
 public:
+	friend class TestNodeResolver; // m_ndef
+
 	void resolveNodeNames();
 
 	content_t test_nr_node1;