strongly encourage the use of fernet, document its limitation (#3623)
diff --git a/docs/fernet.rst b/docs/fernet.rst
index 0c1eb43..65f70cf 100644
--- a/docs/fernet.rst
+++ b/docs/fernet.rst
@@ -163,6 +163,13 @@
For complete details consult the `specification`_.
+Limitations
+-----------
+
+Fernet is ideal for encrypting data that easily fits in memory. As a design
+feature it does not expose unauthenticated bytes. Unfortunately, this makes it
+generally unsuitable for very large files at this time.
+
.. _`Fernet`: https://github.com/fernet/spec/
.. _`specification`: https://github.com/fernet/spec/blob/master/Spec.md