Document the new return value in docstring

src/black/__init__.py

@@ -508,7 +508,13 @@ def get_sources(
     report: "Report",
     stdin_filename: Optional[str],
 ) -> Tuple[Set[Path], Path]:
-    """Compute the set of files to be formatted."""
+    """
+    Compute the set of files to be formatted.
+
+    Returns a tuple, with the first element as a set of paths which are to be
+    checked/formatted by black, and the second element as the project root
+    found out by black.
+    """
     root, method = find_project_root(src)
 
     if verbose:

src/black/files.py

@@ -38,6 +38,11 @@ def find_project_root(srcs: Sequence[str]) -> Tuple[Path, Optional[str]]:
 
     If no directory in the tree contains a marker that would specify it's the
     project root, the root of the file system is returned.
+
+    Returns a tuple with the first element as the project root path and the
+    second element as a string which describes the method by which black
+    found out the project root (None, if `None` of the registered method
+    was used).
     """
     if not srcs:
         srcs = [str(Path.cwd().resolve())]