Add some weasel wording about threaded usage of PGresults.

PGresults used to be read-only from the application's viewpoint, but now
that we've exposed various functions that allow modification of a PGresult,
that sweeping statement is no longer accurate.  Noted by Dmitriy Igrishin.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/1cd1a7c1024503259b3968db6b9e8a888ca97f63

Modified Files
--------------
doc/src/sgml/libpq.sgml |    8 ++++++--
1 files changed, 6 insertions(+), 2 deletions(-)

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