Improve some comments

Sometimes, grep is used to find the TODO comments. Without the proper
explanation, it is hard to know what it refers to (-C / --context may
produce a too large output).

acmed/src/acme_proto/jws.rs

@@ -43,7 +43,7 @@ fn es256_sign(data: &[u8], private_key: &PKey<Private>) -> Result<String, Error>
 }
 
 fn eddsa_ed25519_sign(_data: &[u8], _private_key: &PKey<Private>) -> Result<String, Error> {
-    // TODO: implement
+    // TODO: implement eddsa_ed25519_sign
     Err("EdDSA not implemented.".into())
 }
 

acmed/src/acme_proto/jws/jwk.rs

@@ -33,7 +33,7 @@ impl Es256Jwk {
 
 #[derive(Serialize)]
 pub struct EdDsaEd25519Jwk {
-    // TODO: implement
+    // TODO: implement EdDsaEd25519Jwk
 }
 
 impl EdDsaEd25519Jwk {