Document os.path.ismount() behaviour with btrfs subvolumes.

Author: bionade24

Doc/library/os.path.rst

@@ -298,7 +298,10 @@ the :mod:`glob` module.)
    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.  It is not able to reliably detect bind mounts on the
-   same filesystem.  On Windows, a drive letter root and a share UNC are
+   same filesystem.
+   On Linux systems it will return True for btrfs subvolumes,
+   even if they aren't mountpoints.
+   On Windows, a drive letter root and a share UNC are
    always mount points, and for any other path ``GetVolumePathName`` is called
    to see if it is different from the input path.