Ok,
this is my proposal for the documentation. I hope you like it.

Is this the correct place to post it or can I make a direct pull request?
old:
.. function:: ismount(path)

   Return ``True`` if pathname *path* is a :dfn:`mount point`: a point in a file
   system where a different file system has been mounted.  The function checks
   whether *path*'s parent, :file:`path/..`, is on a different device than *path*,
   or whether :file:`path/..` and *path* point to the same i-node on the same
   device --- this should detect mount points for all Unix and POSIX variants.


new:
.. function:: ismount(path)

   Return ``True`` if pathname *path* is a :dfn:`mount point`: a point in a file
   system where a different file system has been mounted.  The function checks
   whether *path*'s parent, :file:`path/..`, is on a different device than *path*,
   or whether :file:`path/..` and *path* point to the same i-node on the same
   device --- this should detect mount points for all Unix and POSIX variants.
   Note that in Linux not all mountpoints as recognized by the kernel are found this way.
   An example are bind-mounts.
   Also some directories return ``True`` although Linux doesn't recognize them as mount points.
   An example are nested subvolumes in the Btrfs filesystem