manual: KOWN ISSUES: add documentation about the deletions

We currently propagate the deletions if:

- the content of a folder was deleted;
- the parent directory of the folder was deleted.

The second case in incorrect and unexpected. This requires further work.

Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/426
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

docs/offlineimap.known_issues.txt

@@ -1,4 +1,15 @@
 
+* Deletions.
++
+While in usual run the deletions are propagated. To prevent from data loss,
+removing a folder makes offlineimap re-sync the folder. However, propagating the
+removal of the whole content of a folder can happen in the two following cases:
+
+  - The whole content of a folder is deleted but the folder directory still
+    exists.
+
+  - The parent directory of the folder was deleted.
+
 * SSL3 write pending.
 +
 Users enabling SSL may hit a bug about "SSL3 write pending". If so, the