pgsql: Document that Log-Shipping Standby Servers cannot be upgraded by

Document that Log-Shipping Standby Servers cannot be upgraded by
pg_upgrade.

Backpatch to 9.2.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/4810ebe1037e5d9847a0e0ee4ec2fae026ef55f6

Modified Files
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doc/src/sgml/pgupgrade.sgml | 6 ++++++
1 files changed, 6 insertions(+), 0 deletions(-)