Add doc comment and new line at EOF

Author: vcsjones

lib/ssh_data/public_key/sked25519.rb

@@ -12,6 +12,9 @@ def self.algorithm_identifier
         ALGO_SKED25519
       end
 
+      # RFC4253 binary encoding of the public key.
+      #
+      # Returns a binary String.
       def rfc4253
         Encoding.encode_fields(
           [:string, algo],
@@ -29,4 +32,4 @@ def ==(other)
       end
     end
   end
-end
+end