Improve nametrans user documentation

Fill in more details on nametrans and folder filtering. Also give them a
separate section in our user documentation. Everything will be
immediately online at docs.offlineimap.org.

The main change is to describe the reverse nametrans settings that are
needed since 6.4.0 to support remote folder creation.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap.conf

@@ -1,11 +1,14 @@
 # Offlineimap sample configuration file
-# This file documents all possible options and can be quite scary.
-# Looking for a quick start?  Take a look at offlineimap.conf.minimal.
 
-# Settings generally support interpolation. This means values can
-# contain python format strings which refer to other values in the same
-# section, or values in a special DEFAULT section. This allows you for
-# example to use common settings for multiple accounts:
+# This file documents *all* possible options and can be quite scary.
+# Looking for a quick start?  Take a look at offlineimap.conf.minimal.
+# More details can be found in the included user documention, which is
+# also available at: http://docs.offlineimap.org/en/latest/
+
+# NOTE: Settings generally support python interpolation. This means
+# values can contain python format strings which refer to other values
+# in the same section, or values in a special DEFAULT section. This
+# allows you for example to use common settings for multiple accounts:
 #
 # [Repository Gmail1]
 # trashfolder: %(gmailtrashfolder)s
@@ -443,6 +446,10 @@ remoteuser = username
 # value.  I suggest a lambda.  This example below will remove "INBOX." from
 # the leading edge of folders (great for Courier IMAP users)
 #
+# See the user documentation for details and use cases. They are also
+# online at:
+# http://docs.offlineimap.org/en/latest/nametrans.html
+#
 # WARNING: you MUST construct this such that it NEVER returns
 # the same value for two folders, UNLESS the second values are
 # filtered out by folderfilter below.  Failure to follow this rule