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>

src/black/files.py

@@ -59,6 +59,9 @@ def find_project_root(
 ) -> Tuple[Path, str]:
     """Return a directory containing .git, .hg, or pyproject.toml.
 
+    pyproject.toml files are only considered if they contain a [tool.black]
+    section and are ignored otherwise.
+
     That directory will be a common parent of all files and directories
     passed in `srcs`.