bip138: address review

bip-0138.md

@@ -558,9 +558,9 @@ Implementations SHOULD encode and decode the backup using Base64 (RFC 4648).[^ps
 
 ### File Extension
 
-Backups saved to a file SHOULD use the `.bip138` file extension, whether the file
+Backups saved to a file SHOULD use the `.bip138` file extension for the raw binary
-contains the raw binary encoding or its Base64 text representation. A Base64 backup
+encoding and the `.txt` extension for the Base64 text representation, so that the
-MAY instead use the `.txt` extension.
+extension tells implementations which encoding to expect.
 
 ## Rationale