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

Author: bionade24

Doc/library/os.path.rst

@@ -298,9 +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
-   always mount points, and for any other path ``GetVolumePathName`` is called
-   to see if it is different from the input path.
+   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.
 
    .. versionchanged:: 3.4
       Added support for detecting non-root mount points on Windows.