advise singlethreadperfolder when offlineimap hangs

Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

docs/offlineimap.known_issues.txt

@@ -102,6 +102,11 @@ folder behind offlineimap's back, causing them to get assigned a new UID, or
 when offlineimap first syncs a pre-existing Maildir. In the latter case, it
 could appear as if a noticeable and random subset of old messages are synced.
 
+* Offlineimap hangs.
++
+When having unexpected hangs it's advised to set `singlethreadperfolder' to
+'yes', especially when in IMAP/IMAP mode (no maildir).
+
 * Passwords in netrc.
 +
 Offlineimap doesn't know how to retrieve passwords when more than one account is

offlineimap.conf

@@ -1080,7 +1080,8 @@ remoteuser = username
 # download in UIDs order.
 #
 # If this is unset (the default), then up to maxconnections threads are used
-# across all currently syncing folders.
+# across all currently syncing folders. This option is sightly recommended in
+# IMAP/IMAP mode (no maildir).
 #
 #singlethreadperfolder = no