rustdoc: summary line for hg_path_to_os_string
The main motivation of this change is to avoid the TODO being
the summary line, even though this leads to a pretty obvious summary.
Then doc-comments for the other functions are introduced for
consistency.
[build-system]
requires = ["setuptools", "wheel"]
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true