Document that scdc doesn't implement transaction guarantees.

scdc/README.md

@@ -10,6 +10,8 @@ This crate provides `ScdBackend`/`ScdTransaction`, which is an implementation of
 [scdaemon](https://www.gnupg.org/documentation/manuals/gnupg/Invoking-SCDAEMON.html)
 to access OpenPGP cards.
 
+Note that (unlike `openpgp-card-pcsc`), this backend doesn't implement transaction guarantees.
+
 **Known limitations**
 
 - Uploading RSA 4096 keys via `scdaemon` doesn't work with cards that don't 

scdc/src/lib.rs

@@ -4,6 +4,8 @@
 //! This crate implements the experimental `ScdBackend`/`ScdTransaction` backend for the
 //! `openpgp-card` crate.
 //! It uses GnuPG's scdaemon (via GnuPG Agent) to access OpenPGP cards.
+//!
+//! Note that (unlike `openpgp-card-pcsc`), this backend doesn't implement transaction guarantees.
 
 use futures::StreamExt;
 use lazy_static::lazy_static;