clarify DEWIF Format chapter and Base64 usage

rfc/0013_Duniter_Encrypted_Wallet_Import_Format.md

@@ -33,8 +33,9 @@ So currently a public key for DUBP is to be understood as an Ed25519 public key.
 
 DEWIF is an inter-operable and secure format for any DUBP wallet.
 
-DEWIF format is a base64 string.  
-This base64 string encodes a sequence of bytes whose structure is defined below.
+DEWIF format is a sequence of bytes whose structure is defined below. It can be used as raw binary data where suitable.  
+
+To use the binary data in a text context (email, ftp text transfer, json, html, etc) it is recommended to encode the binary data in a Base64 string. The Base64 string is used here to check the binary data validity. 
 
 ## Dewif bytes structure