write some words about encrypt/decrypt in fernet (#3756)

diff --git a/docs/fernet.rst b/docs/fernet.rst
index 65f70cf..82d94fa 100644
--- a/docs/fernet.rst
+++ b/docs/fernet.rst
@@ -37,6 +37,9 @@
 
     .. method:: encrypt(data)
 
+        Encrypts data passed. The result of this encryption is known as a
+        "Fernet token" and has strong privacy and authenticity guarantees.
+
         :param bytes data: The message you would like to encrypt.
         :returns bytes: A secure message that cannot be read or altered
                         without the key. It is URL-safe base64-encoded. This is
@@ -52,6 +55,11 @@
 
     .. method:: decrypt(token, ttl=None)
 
+        Decrypts a Fernet token. If successfully decrypted you will receive the
+        original plaintext as the result, otherwise an exception will be
+        raised. It is safe to use this data immediately as Fernet verifies
+        that the data has not been tampered with prior to returning it.
+
         :param bytes token: The Fernet token. This is the result of calling
                             :meth:`encrypt`.
         :param int ttl: Optionally, the number of seconds old a message may be