[3.14] gh-81520: Document unexpected `os.path.ismount` behaviour with btrfs subvolumes (GH-136058) (GH-136471)

gh-81520: Document unexpected `os.path.ismount` behaviour with btrfs subvolumes (GH-136058)
(cherry picked from commit 591abcc01fcf1c65c7fdfaca7274f5d3f9f022da)

Co-authored-by: Oskar Roesler <o.roesler@oscloud.info>
1 file changed