Bring GMail labels section on the manual up to date

 - there is no need to set SQlite backend anymore;
 - explain a bit more that some headers are recognized
   by email clients.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>

docs/MANUAL.rst

@@ -438,18 +438,20 @@ Sync from Gmail to a local Maildir with labels
 
 This is an example of a setup where GMail gets synced with a local Maildir.
 It also keeps track of GMail labels, that get embedded into the messages
-under the header X-Keywords (or whatever labelsheader is set to), and syncs
-them back and forth the same way as flags.
+under the header configured in labelsheader, and syncs them back and forth
+the same way as flags.
+The header used for the labels may need to be set according to the email
+client used.
+Some choices that may be recognized by email clients are `X-Keywords` or `X-Labels`.
 
-The first time it runs on a large repository may take some time as the labels
-are read / embedded on every message.  Afterwards local label changes are detected
-using modification times (much faster)::
+The first time OfflineIMAP runs with synclabels enabled on a large repository it
+may take some time as the labels are read / embedded on every message.
+Afterwards local label changes are detected using modification times, which is
+much faster::
 
     [Account Gmail-mine]
     localrepository = Gmaillocal-mine
     remoterepository = Gmailserver-mine
-    # Need this to be able to sync labels
-    status_backend = sqlite
     synclabels = yes
     # This header is where labels go.  Usually you will be fine
     # with default value, but in case you want it different,