doc: Update doc about journalling layer

Documentation of journalling layer in
Documentation/DocBook/filesystems.tmpl speaks about JBD layer. Since
that is going away, update the documentation to speak about JBD2. Also
update the parts that have changed since someone last touched the
document and remove some parts which are just misleading and outdated.

Signed-off-by: Jan Kara <jack@suse.com>