doc: clarify authenticity of plaintexts in update

PR-URL: #53784
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>

doc/api/crypto.md

@@ -978,6 +978,11 @@ The `decipher.update()` method can be called multiple times with new data until
 [`decipher.final()`][] is called. Calling `decipher.update()` after
 [`decipher.final()`][] will result in an error being thrown.
 
+Even if the underlying cipher implements authentication, the authenticity and
+integrity of the plaintext returned from this function may be uncertain at this
+time. For authenticated encryption algorithms, authenticity is generally only
+established when the application calls [`decipher.final()`][].
+
 ## Class: `DiffieHellman`
 
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