update docs to explain tag requirements and valueerror
diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index 8d8d558..4614868 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -349,8 +349,8 @@
Do not reuse an ``initialization_vector``
with a given ``key``.
- :param bytes tag: The tag bytes to verify during decryption. Must be provided
- for decryption, but is ignored when encrypting.
+ :param bytes tag: The tag bytes to verify during decryption. When encrypting
+ this must be None.
.. doctest::