doc: clarify diffieHellman.generateKeys recomputes same key

Clarify that calling generateKeys() after a private key has been
set recomputes the same public key deterministically, rather than
implying the public key changes. The public key is fully determined
by the private key, so the result is identical unless the private
key was changed via setPrivateKey().

Fixes: #56990

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kovan

doc/api/crypto.md

@@ -1121,8 +1121,10 @@ If `encoding` is provided a string is returned; otherwise a
 [`Buffer`][] is returned.
 
 This function is a thin wrapper around [`DH_generate_key()`][]. In particular,
-once a private key has been generated or set, calling this function only updates
-the public key but does not generate a new private key.
+once a private key has been generated or set, calling this function only
+recomputes the public key from the existing private key. Since the public key is
+determined by the private key, the result will be the same unless the private key
+has been changed via [`diffieHellman.setPrivateKey()`][].
 
 ### `diffieHellman.getGenerator([encoding])`
 
@@ -6572,6 +6574,7 @@ See the [list of SSL OP Flags][] for details.
 [`decipher.final()`]: #decipherfinaloutputencoding
 [`decipher.update()`]: #decipherupdatedata-inputencoding-outputencoding
 [`diffieHellman.generateKeys()`]: #diffiehellmangeneratekeysencoding
+[`diffieHellman.setPrivateKey()`]: #diffiehellmansetprivatekeyprivatekey-encoding
 [`diffieHellman.setPublicKey()`]: #diffiehellmansetpublickeypublickey-encoding
 [`ecdh.generateKeys()`]: #ecdhgeneratekeysencoding-format
 [`ecdh.setPrivateKey()`]: #ecdhsetprivatekeyprivatekey-encoding