add AESGCM AEAD support (#3785)

* add AESGCM AEAD support

* remove stray newline

* move AESGCM docs above CCM
diff --git a/docs/hazmat/primitives/symmetric-encryption.rst b/docs/hazmat/primitives/symmetric-encryption.rst
index 7e05acd..9e27540 100644
--- a/docs/hazmat/primitives/symmetric-encryption.rst
+++ b/docs/hazmat/primitives/symmetric-encryption.rst
@@ -294,6 +294,11 @@
 
     .. danger::
 
+        If you are encrypting data that can fit into memory you should strongly
+        consider using
+        :class:`~cryptography.hazmat.primitives.ciphers.aead.AESGCM` instead
+        of this.
+
         When using this mode you **must** not use the decrypted data until
         the appropriate finalization method
         (:meth:`~cryptography.hazmat.primitives.ciphers.CipherContext.finalize`