PyDoc: improve mathutils python signatures and doc-strings

- Adding `/` to signatures to indicate that arguments are accepted only
  as positional.
- Document default values and optional arguments in consistent way with
  other docs.
- Use `Literal` for strings to indicate set of allowed string values
  (later it can also be used in typing stubs).
- Document some undocumented arguments.

Ref !143278

source/blender/python/mathutils/mathutils_kdtree.cc

@@ -402,6 +402,9 @@ PyDoc_STRVAR(
     py_KDtree_doc,
     "KdTree(size) -> new kd-tree initialized to hold ``size`` items.\n"
     "\n"
+    "   :arg size: Number of items.\n"
+    "   :type size: int\n"
+    "\n"
     ".. note::\n"
     "\n"
     "   :class:`KDTree.balance` must have been called before using any of the ``find`` "