Use `-C' to prevent information leaks

docs/ed25519.md

@@ -23,7 +23,7 @@ Ed25519 is the best recommendation in 2019.
 
 ## Generate an Ed25519 key
 
-    ssh-keygen -a 100 -t ed25519 -f ~/.ssh/id_ed25519
+    ssh-keygen -a 100 -t ed25519 -f ~/.ssh/id_ed25519 -C ''
 
 Switches:
 
@@ -36,6 +36,8 @@ Switches:
   discovered automatically by the SSH agent, it must be stored in the default
   `.ssh` directory within your home directory. There are other strategies for
   organizing specific machine keys not covered in this guide.
+- **-C:** Sets the comment. If left unset the comment can leak your local
+  username.
 
 When asked to enter a passphrase, do so. An ssh key without a passphrase is
 vulnerable if stolen.