doc: clarify DiffieHellmanGroup class docs

Make it clearer in the docs that DiffieHellmanGroup does
not support changing the keys after creation.

PR-URL: #38363
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Adrian Estrada <edsadr@gmail.com>

doc/api/crypto.md

@@ -735,8 +735,10 @@ module):
 added: v0.7.5
 -->
 
-The `DiffieHellmanGroup` class takes a well-known modp group as its argument but
-otherwise works the same as `DiffieHellman`.
+The `DiffieHellmanGroup` class takes a well-known modp group as its argument.
+It works the same as `DiffieHellman`, except that it does not allow changing
+its keys after creation. In other words, it does not implement `setPublicKey()`
+or `setPrivateKey()` methods.
 
 ```js
 const name = 'modp1';