pg_upgrade docs

I have created the attached patch which rewords the mention of
pg_upgrade in the docs.  I think it is clearer and more concise.  Should
we mention that people should have a recent pg_dump?  If so, that should
be in the pg_upgrade man page, not here.

-- 
  Bruce Momjian          http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +
diff --git a/doc/src/sgml/backup.sgml b/doc/src/sgml/backup.sgml
index c14ae43..123e769 100644
*** a/doc/src/sgml/backup.sgml
--- b/doc/src/sgml/backup.sgml
*************** psql -f backup postgres
*** 1565,1578 ****
     Other data migration methods
  
    
!    The contrib</> program
!    pg_upgrade
!    allows an installation to be migrated in-place from one major
!    PostgreSQL</> version to the next.  Keep in mind that this
!    method does not provide any scope for running old and new versions
!    concurrently.  Also, pg_upgrade is much less
!    battle-tested than pg_dump, so having an
!    up-to-date backup is strongly recommended in case something goes wrong.
    
  
    
--- 1565,1575 ----
     Other data migration methods
  
    
!    The pg_upgrade module allows an
!    installation to be migrated in-place from one major
!    PostgreSQL</> version to the next.  Upgrades can be
!    performed in minutes, though the server cannot be accessed during
!    the upgrade.