[3.13] gh-46236: Add docs for PyUnicode_GetDefaultEncoding() doc (GH-130335) (GH-130511)

* Clarify sys.getdefaultencoding() documentation

* Add missing documentation for PyUnicode_GetDefaultEncoding,
  the C equivalent of sys.getdefaultencoding
(cherry picked from commit 9f25c1f012c8d432a93bf2dcad5f19a64dc00d3c)

Co-authored-by: RUANG (James Roy) <longjinyii@outlook.com>