Clarify encryption of key for borg key export (#3297)

clarify / fix docs for borg key export, fixes #3296

src/borg/archiver.py

@@ -2497,11 +2497,15 @@ class Archiver:
         key_export_epilog = process_epilog("""
         If repository encryption is used, the repository is inaccessible
         without the key. This command allows to backup this essential key.
+        Note that the backup produced does not include the passphrase itself
+        (i.e. the exported key stays encrypted). In order to regain access to a
+        repository, one needs both the exported key and the original passphrase.
 
-        There are two backup formats. The normal backup format is suitable for
+        There are three backup formats. The normal backup format is suitable for
         digital storage as a file. The ``--paper`` backup format is optimized
         for printing and typing in while importing, with per line checks to
-        reduce problems with manual input.
+        reduce problems with manual input. The ``--qr-html`` creates a printable
+        HTML template with a QR code and a copy of the ``--paper``-formatted key.
 
         For repositories using keyfile encryption the key is saved locally
         on the system that is capable of doing backups. To guard against loss