Fix missing Sphinx API references

[Gonghan-Princess]

Fixes #24.

Strict Sphinx HTML and man-page builds currently emit 12 warnings, including unresolved API references. Document the three NearestDict rounding constants so they have real targets, link SortedDict to its external API documentation, and give the two custom pickle reducers docstrings that link to their own constructors.

Document OrderedDict.update explicitly with its valid signature and behavior. This replaces the malformed autodoc entry generated from MutableMapping.update while preserving the public method's anchor and documentation. Also set the documentation language to English instead of None.

Validation:

• Both builders failed before the change with 12 warnings each; sphinx-build -n -W --keep-going -b html and -b man now pass with Sphinx 9.1.0.
• Verified the repaired constant, constructor, and method targets, plus the external class link. Package ASTs are unchanged after removing docstrings.
• 64 tests passed with 100% coverage on Python 3.12; doc8 passed on an LF export, isort passed, and pylint exited successfully.

Current Blue, flake8, and mypy checks expose pre-existing issues also reproduced in the untouched baseline. An additional link audit found six unchanged source-view backlinks unrelated to this issue. These are left outside this documentation fix.

AI assistance: OpenAI Codex helped reproduce the warnings, prepare the documentation changes, verify generated links, run checks, and draft this description.