PyDoc: use type-hint syntax, add missing docs & some corrections

sphinx_doc_gen.py:
- Add _PRIVATE_ATTR_INCLUDE so underscore-prefixed types can be
  selectively included in API docs (needed to reference return values).
- Add :return:/:rtype: for operators.
- Fix context type entries (AnnotationLayer, FluidModifier, Curves).

bpy.props:
- Add :return:/:rtype: for property declaration functions.

gpu:
- Expose MatrixStackContext and OffScreenStackContext as documented
  gpu.types so :rtype: references resolve.

bmesh.ops:
- Use Sequence[float] for vector input params.
- Add default_value for single-element BMO_OP_SLOT_ELEMENT_BUF slots.

bpy.types:
- Convert `tuple of X` to `tuple[X, ...]` and `list of X` to `list[X]`.
- Add :return:/:rtype: to Context.path_resolve and Operator.as_keywords.
- Fix Menu example link reference.

bpy_extras:
- Convert Python type annotations to :param:/:type:/:rtype:,
  add missing doc-strings.

bpy.app.handlers:
- Add :type: with callable signatures.
- Fix depsgraph_update handler description
  (the second argument is optional).
- Fix missing argument descriptions for render and undo/redo handlers.

bpy.context
- Use type hints for for property listing.

mathutils.kdtree:
- Fix doc-string syntax.
- Clarify that the size is an upper limit.

Various minor corrections to other modules.

doc/python_api/examples/bpy.types.Menu.0.py

@@ -15,7 +15,7 @@ convention for menus.
 .. note::
 
    Menus have their :class:`UILayout.operator_context` initialized as
-   'EXEC_REGION_WIN' rather than 'INVOKE_REGION_WIN' (see :ref:`Execution Context <operator-execution_context>`).
+   'EXEC_REGION_WIN' rather than 'INVOKE_REGION_WIN' (see :ref:`Execution Context <rna_enum_operator_context_items>`).
    If the operator context needs to initialize inputs from the
    :class:`Operator.invoke` function, then this needs to be explicitly set.
    When a menu is added to UI elements such as a panel or header,