pgsql: Marginal improvements to documentation of plpgsql's OPEN cursor

Marginal improvements to documentation of plpgsql's OPEN cursor statement.

Rearrange text to improve clarity, and add an example of implicit reference
to a plpgsql variable in a bound cursor's query.  Byproduct of some work
I'd done on the "named cursor parameters" patch before giving up on it.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/0898d71f66ed884af726556ac9ffc8081dddc757

Modified Files
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doc/src/sgml/plpgsql.sgml |   44 ++++++++++++++++++++++++++++----------------
1 files changed, 28 insertions(+), 16 deletions(-)