doc: clarify DiffieHellmanGroup class docs

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

doc/api/crypto.md

@@ -1114,8 +1114,10 @@ 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.
+Works the same as `DiffieHellman`, except that it does not allow changing its
+keys after creation - i.e. does not implement `setPublicKey` or `setPrivateKey`
+methods.
 
 ```mjs
 const { createDiffieHellmanGroup } = await import('crypto');