pgsql: Backpatch to 9.0 a doc mention that a BBU does not prevent parti

Backpatch to 9.0 a doc mention that a BBU does not prevent partial page
writes.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=c47458529533bc88e34778818a4ce7d5c309d7a8

Modified Files
--------------
doc/src/sgml/wal.sgml | 10 ++++++----
1 files changed, 6 insertions(+), 4 deletions(-)