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_interpolate.cc

@@ -30,7 +30,7 @@ PyDoc_STRVAR(
 PyDoc_STRVAR(
     /* Wrap. */
     M_Interpolate_poly_3d_calc_doc,
-    ".. function:: poly_3d_calc(veclist, pt)\n"
+    ".. function:: poly_3d_calc(veclist, pt, /)\n"
     "\n"
     "   Calculate barycentric weights for a point on a polygon.\n"
     "\n"