doc: wrap the help text for the purge extension
Not sure why `black` or some other checker didn't complain about this.
Differential Revision: https://phab.mercurial-scm.org/D11036
[build-system]
requires = ["setuptools", "wheel"]
build-backend = "setuptools.build_meta"
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true