pgsql: upgrade docs: highlight pg_upgrade, warn about globals preserva

upgrade docs: highlight pg_upgrade, warn about globals preservation

Also, remove OID preservation mention, mention non-text dump formats

Backpatch through 9.4

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/bf1c866597a0e59a30b13a66f3ae505655b61853

Modified Files
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doc/src/sgml/backup.sgml | 27 ++++++++++++++------------
doc/src/sgml/runtime.sgml | 47 ++++++++++++++++++++++++++++-----------------
2 files changed, 44 insertions(+), 30 deletions(-)

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