Clarify in a comment that __del__ is not always called, so temporary...

Clarify in a comment that __del__ is not always called, so temporary directories are not neccessary removed the *moment* they go out of scope.

At least in CPython...

Ref: #234

diffoscope/tempfiles.py

@@ -47,10 +47,12 @@ def get_temporary_directory(*args, **kwargs):
     statement. Otherwise it's cleaned up when the object is garbage collected.
 
     WARNING, don't do this:
+
         tmpdir = get_temporary_directory().name
 
-    This will result in the immediate removal of the directory, as there is no
-    reference pointing to the TemporaryDirectory object.
+    ... as this may result in the removal of the directory at some unexpected
+    time in the future. (Or not at all, depending on whether there are other,
+    unexpected, references to this instance.)
     """
     kwargs["dir"] = kwargs.pop("dir", _get_base_temporary_directory())