sandbox-permissions: Document that fs is tmpfs

docs/sandbox-permissions.rst

@@ -172,11 +172,11 @@ Other filesystem access guidelines include:
 
  For example, if an application hardcodes the directory ``~/.foo``,
  without any ``home`` access and no ``--persist`` the directory will be
- lost from the sandbox once exited. A ``--persist=.foo``
- bind mounts ``~/.foo`` `inside the sandbox` to
- ``~/.var/app/$FLATPAK_ID/.foo`` on host thus allowing an app to
- persistently store data in ``~/.var/app/$FLATPAK_ID/.foo`` which
- would otherwise be lost.
+ lost from the sandbox once exited due to the filesystem being set up
+ as tmpfs by flatpak unless overriden. A ``--persist=.foo`` bind mounts
+ ``~/.foo`` `inside the sandbox` to ``~/.var/app/$FLATPAK_ID/.foo`` on
+ host thus allowing an app to persistently store data in
+ ``~/.var/app/$FLATPAK_ID/.foo`` which would otherwise be lost.
 
  A ``--persist=.`` will `persist` all directories.