document some breakage

TODO

@@ -17,6 +17,9 @@ clarify error cases of transactions.
 
 cleanup/improve error messages in the drivers.
 
+sync.c does not consider UID 0 valid, which is wrong. fixing this requires
+an incompatible change or a remapping hack in sync state files.
+
 clarify UID 0 vs. < 0 returns vor store_msg & find_msg.
 
 create dbg_srec(srec, fmt, ...).

src/mbsync.1

@@ -452,11 +452,15 @@ times within a Group.
 .SH INHERENT PROBLEMS
 Changes done after \fBmbsync\fR has retrieved the message list will not be
 synchronised until the next time \fBmbsync\fR is invoked.
-.P
+..
+.SH BUGS
 Using \fBTrash\fR on IMAP Stores bears a race condition: messages will be
 lost if they are marked as deleted after the message list was retrieved but
-before the mailbox is expunged. There is no risk as long as the IMAP mailbox
+before the mailbox is expunged. This can be fixed by using UID EXPUNGE.
-is not simultaneously accessed by \fBmbsync\fR and another mail client.
+There is no risk as long as the IMAP mailbox is not simultaneously accessed
+by \fBmbsync\fR and another mail client.
+.P
+The IMAP message UID 0 is assumed to be impossible, which is wrong.
 ..
 .SH FILES
 .TP