Recognize configuration for idlefolders

Mark this option as experimental and document its shortcomings in
MANUAL.rst.

This code was originally by James Bunton <jamesbunton@fastmail.fm>.

Signed-off-by: Ethan Glasser-Camp <ethan@betacantrips.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap.conf

@@ -364,6 +364,23 @@ remoteuser = username
 #
 # reference = Mail
 
+# In between synchronisations, OfflineIMAP can monitor mailboxes for new
+# messages using the IDLE command. If you want to enable this, specify here
+# the folders you wish to monitor. Note that the IMAP protocol requires a
+# separate connection for each folder monitored in this way, so setting
+# this option will force settings for:
+#     maxconnections - to be at least the number of folders you give
+#     holdconnectionopen - to be true
+#     keepalive - to be 29 minutes unless you specify otherwise
+#
+# This feature isn't complete and may well have problems. BE AWARE THIS
+# IS EXPERIMENTAL STUFF.  See the manual for more details.
+#
+# This option should return a Python list. For example
+#
+# idlefolders = ['INBOX', 'INBOX.Alerts']
+#
+
 # OfflineIMAP can use multiple connections to the server in order
 # to perform multiple synchronization actions simultaneously.
 # This may place a higher burden on the server.  In most cases,