sandbox-permissions: Add /var/tmp location for TMPDIR alternative

docs/sandbox-permissions.rst

@@ -187,7 +187,9 @@ Other filesystem access guidelines include:
  directory, and can be useful for applications that hardcode file paths
  in ``~/``.
 - If an application uses ``$TMPDIR`` to contain lock files you may want to
- add a wrapper script that sets it to ``$XDG_RUNTIME_DIR/app/$FLATPAK_ID``.
+ add a wrapper script that sets it to
+ ``$XDG_RUNTIME_DIR/app/$FLATPAK_ID`` (tmpfs) or ``/var/tmp`` (persistent
+ on host).
 - Retaining and sharing configuration with non-Flatpak installations is to
  be avoided.