VoxelArea: add_{x,y,z,p} must be static

Fix some documentations issues
Use getNodeNoCheck(v3s16, ...) in some cases instead of getNodeNoCheck(x, y, z, ...)

src/mapgen/mapgen_v7.cpp

@@ -539,7 +539,7 @@ int MapgenV7::generateTerrain()
 					vm->m_data[vi] = n_air;
 				}
 			}
-			vm->m_area.add_y(em, vi, 1);
+			VoxelArea::add_y(em, vi, 1);
 			index3d += ystride;
 		}
 	}