[3.11] gh-83006: Document behavior of shutil.disk_usage for non-mounted filesystems on Unix (GH-107031) (#107048)

(cherry picked from commit 6e5f223)

Co-authored-by: Matthieu Caneill <matthieucan@users.noreply.github.com>

Author: miss-islington

Doc/library/shutil.rst

@@ -393,6 +393,12 @@ Directory and files operations
    total, used and free space, in bytes. *path* may be a file or a
    directory.
 
+   .. note::
+
+      On Unix filesystems, *path* must point to a path within a **mounted**
+      filesystem partition. On those platforms, CPython doesn't attempt to
+      retrieve disk usage information from non-mounted filesystems.
+
    .. versionadded:: 3.3
 
    .. versionchanged:: 3.8

Misc/NEWS.d/next/Library/2023-07-22-15-51-33.gh-issue-83006.21zaCz.rst

@@ -0,0 +1,2 @@
+Document behavior of :func:`shutil.disk_usage` for non-mounted filesystems
+on Unix.