pgsql: Update pg_upgrade docs to mention its use in a less risk-warning

Update pg_upgrade docs to mention its use in a less risk-warning way,
and update the pg_upgrade docs to mention its reliance on no changes to
the storage format (the later based on Robert Haas's patch).

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=49450f01eca39308989cf668f60b7c345e72fc7b

Modified Files
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doc/src/sgml/pgupgrade.sgml |   16 +++++++++++++---
doc/src/sgml/runtime.sgml   |   14 +++++---------
2 files changed, 18 insertions(+), 12 deletions(-)