add note regarding not truncating tags
diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index f4d0457..8ed64c7 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -352,6 +352,11 @@
Do not reuse an ``initialization_vector``
with a given ``key``.
+ .. note::
+
+ Do not truncate the GCM authentication tag unless absolutely necessary.
+ If you must truncate the minimum allowable length is 4 bytes.
+
:param bytes tag: The tag bytes to verify during decryption. When encrypting
this must be None.