docs: change-passphrase only changes the passphrase, fixes #2990

(cherry picked from commit b6abee4d6852f2b8b569228e45751e1ae8dc96b3)

borg/archiver.py

@@ -1257,6 +1257,11 @@ class Archiver:
         change_passphrase_epilog = textwrap.dedent("""
         The key files used for repository encryption are optionally passphrase
         protected. This command can be used to change this passphrase.
+
+        Please note that this command only changes the passphrase, but not any
+        secret protected by it (like e.g. encryption/MAC keys or chunker seed).
+        Thus, changing the passphrase after passphrase and borg key got compromised
+        does not protect future (nor past) backups to the same repository.
         """)
         subparser = subparsers.add_parser('change-passphrase', parents=[common_parser],
                                           description=self.do_change_passphrase.__doc__,