Arch Linux: Remove DropBear key limitation note

ed25519 support: mkj/dropbear#91

Closes #131

Signed-off-by: Vlad Petrov <ejiek@mail.ru>
Signed-off-by: Maurice Zhou <ja@apvc.uk>
[more concise wording; fix rst formatting]
Signed-off-by: Richard Laager <rlaager@wiktel.com>

docs/Getting Started/Arch Linux/Arch Linux Root on ZFS.rst

@@ -749,8 +749,6 @@ Optional Configuration
 
       vi /etc/dropbear/root_key
 
-     Note that dropbear only supports RSA keys.
-
   #. Edit mkinitcpio::
 
       tee /etc/mkinitcpio.conf <<- 'EOF'
@@ -769,6 +767,17 @@ Optional Configuration
 
       ssh-keygen -Am pem
 
+  #. Manually convert ed25519 host key to dropbear
+     -readable format::
+
+      dropbearconvert openssh dropbear /etc/ssh/ssh_host_ed25519_key /etc/dropbear/dropbear_ed25519_host_key
+
+     `mkinitcpio-dropbear
+     <https://archlinux.org/packages/community/any/mkinitcpio-dropbear/>`__,
+     lacks support for converting ed25519 host key,
+     `see this pull request
+     <https://github.com/grazzolini/mkinitcpio-dropbear/pull/13>`__.
+
   #. Regenerate initrd::
 
       mkinitcpio -P