Add a new Mode interface to document mode.name and start on some prose docs for interfaces.
diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index 7d3b072..4b37d39 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -149,6 +149,9 @@
:param bytes key: The secret key, 32-448 bits in length (in increments of
8). This must be kept secret.
+
+.. _symmetric-encryption-modes:
+
Modes
~~~~~