Update IMAP documentation.

Update the IMAP documentation to make it clear that storing voicemails
in the same folder as a large number of emails could potentially cause
significant slow downs when writing or retrieving voicemails.

(closes issue #16704)
Reported by: TimeHider
Tested by: lmadsen, TimeHider

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@250050 65c4cc65-6c06-0410-ace0-fbb531ad65f3

doc/imapstorage.txt

@@ -144,6 +144,12 @@ IMAP folders at the same level of hierarchy as the INBOX.  These will be
 used as alternate folders for storing voicemail messages to mimic the 
 behavior of the current (file-based) voicemail system.
 
+Please note that it is not recommended to store your voicemails in the top
+level folder where your users will keep their emails, especially if there 
+are a large number of emails. A large number of emails in the same folder(s)
+that you're storing your voicemails could cause a large delay as Asterisk must
+parse through all the emails. For example a mailbox with 100 emails in it could
+take up to 60 seconds to receive a response.
 
 ==================================
 Separate vs. Shared Email Accounts