geometry_creation.rst: add sections for assemblies, clipping normals, openings

src/ifcopenshell-python/docs/ifcopenshell-python/geometry_creation.rst

@@ -775,3 +775,52 @@ responsibility to make sure the geometry is correct.
 
     # Assign our new body geometry back to our beam
     ifcopenshell.api.geometry.assign_representation(model, product=beam, representation=representation)
+
+Moving assemblies
+-----------------
+
+When moving an assembly and you want all children to follow, pass
+``should_transform_children=True``. The default (``False``) rewrites each
+child's local placement to preserve its world position, so the parent moves
+but the children stay where they are.
+
+.. code-block:: python
+
+    matrix = numpy.eye(4)
+    matrix[:,3][0:3] = (0, 0, 6)
+
+    # Move the assembly; children travel with it.
+    ifcopenshell.api.geometry.edit_object_placement(model,
+        product=assembly, matrix=matrix, is_si=True,
+        should_transform_children=True)
+
+Clipping normals convention
+---------------------------
+
+The ``normal`` passed to :func:`geometry.clip_solid`,
+:func:`geometry.clip_solid_bounded`, and the ``clippings`` parameter of
+:func:`geometry.add_wall_representation` points toward the **removed**
+material (the discarded side), not toward the kept material.
+
+.. code-block:: python
+
+    # Clip the top of a wall to a lean-to slope.
+    # normal points upward into the wedge that will be removed.
+    bcr = ifcopenshell.api.geometry.clip_solid(model,
+        item=extrusion,
+        location=[0.0, 0.0, 3.26],
+        normal=[0.419, 0.0, 0.908])
+    shape_representation.RepresentationType = "Clipping"
+
+Opening lifecycle
+-----------------
+
+``feature.remove_feature`` permanently deletes the feature entity from the
+model. Any fillings (windows, doors) that occupied the opening become
+orphaned and must be separately removed via ``root.remove_product``.
+
+.. code-block:: python
+
+    # Remove a window and its opening from a wall.
+    ifcopenshell.api.root.remove_product(model, product=window)
+    ifcopenshell.api.feature.remove_feature(model, feature=opening)