Document `find_project_root` ignoring `pyproject.toml` without `[tool.black]` (#4425)

Extend the docstring of black's `find_project_root` to mention that it ignores
`pyproject.toml` files without a `[tool.black]` section.

This is relevant because that function is also used by other python packages
that use black. I found that e.g. datamodel-code-generator [1] uses that
function and that there the ignoring of the pyproject.toml files lead to
a degradation [2]. I think in that case it would be better to not use black's function
for finding the pyproject.toml, but in any case this behavior should be documented.

1: https://github.com/koxudaxi/datamodel-code-generator
2: https://github.com/koxudaxi/datamodel-code-generator/issues/2052



Co-authored-by: Michael Eliachevitch <Michael.Eliachevitch@blueyonder.com>