Add warning about interrupting copy

libraries/liblmdb/mdb_copy.1

@@ -17,6 +17,12 @@ If
 is specified it must be the path of an empty directory
 for storing the backup. Otherwise, the backup will be
 written to stdout.
+
+Note: currently, if the copy is interrupted a stale lock
+will be left in the LMDB environment. This may be fixed
+in a future release, but until then you must not
+interrupt the copy process.
+
 .SH DIAGNOSTICS
 Exit status is zero if no errors occur.
 Errors result in a non-zero exit status and