Python API Docs: add an example of `Bone.convert_local_to_pose` usage.

The use case for this method is quite obscure and difficult to
understand without an example. Despite how big looks, this is
actually the simplest example that makes sense.

doc/python_api/examples/bpy.types.Bone.convert_local_to_pose.py

@@ -0,0 +1,40 @@
+"""
+This method enables conversions between Local and Pose space for bones in
+the middle of updating the armature without having to update dependencies
+after each change, by manually carrying updated matrices in a recursive walk.
+"""
+
+def set_pose_matrices(obj, matrix_map):
+    "Assign pose space matrices of all bones at once, ignoring constraints."
+
+    def rec(pbone, parent_matrix):
+        matrix = matrix_map[pbone.name]
+
+        ## Instead of:
+        # pbone.matrix = matrix
+        # bpy.context.view_layer.update()
+
+        # Compute and assign local matrix, using the new parent matrix
+        if pbone.parent:
+            pbone.matrix_basis = pbone.bone.convert_local_to_pose(
+                matrix,
+                pbone.bone.matrix_local,
+                parent_matrix=parent_matrix,
+                parent_matrix_local=pbone.parent.bone.matrix_local,
+                invert=True
+            )
+        else:
+            pbone.matrix_basis = pbone.bone.convert_local_to_pose(
+                matrix,
+                pbone.bone.matrix_local,
+                invert=True
+            )
+
+        # Recursively process children, passing the new matrix through
+        for child in pbone.children:
+            rec(child, matrix)
+
+    # Scan all bone trees from their roots
+    for pbone in obj.pose.bones:
+        if not pbone.parent:
+            rec(pbone, None)