Doc: ~/.config/borg/keys is not used w/ repokey (#6185)

docs: ~/.config/borg/keys is not used for repokey keys, fixes #6107

Thanks to ThomasWaldmann for suggesting this
https://github.com/borgbackup/borg/pull/6185#discussion_r793995379

docs/faq.rst

@@ -514,7 +514,8 @@ The Borg config directory has content that you should take care of:
   recovered.
 
 ``keys`` subdirectory
-  In this directory all your repository keyfiles are stored. You MUST make sure to have an
+  All your borg keyfile keys are stored in this directory. Please note that
+  borg repokey keys are stored inside the repository. You MUST make sure to have an
   independent backup of these keyfiles, otherwise you cannot access your backups anymore if you lose
   them. You also MUST keep these files secret; everyone who gains access to your repository and has
   the corresponding keyfile (and the key passphrase) can extract it.