tor-dirmgr: Warn on a bug case.

This warning occurs if we ask for microdescriptors from our local
cache, and our cache gives us something we didn't ask for.  It
shouldn't be possible, so let's warn when it occurs.

This patch resolves an XXXX.

crates/tor-dirmgr/src/state.rs

@@ -611,8 +611,7 @@ impl<DM: WriteNetDir> DirState for GetMicrodescsState<DM> {
         for (id, text) in docs {
             if let DocId::Microdesc(digest) = id {
                 if !self.missing.remove(&digest) {
-                    // XXXX BUG:
-                    // we didn't want this.
+                    warn!("Bug: loaded a microdesc that we didn't want from the cache.");
                     continue;
                 }
                 if let Ok(md) = Microdesc::parse(text.as_str()?) {