pgsql: PL/Python: Improve documentation of nrows() method

PL/Python: Improve documentation of nrows() method

Clarify that nrows() is the number of rows processed, versus the
number of rows returned, which can be obtained using len. Also add
tests about that.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/0f48e0675134eccd905eaf696a03c1e8cc85eab4

Modified Files
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doc/src/sgml/plpython.sgml | 7 +++-
src/pl/plpython/expected/plpython_spi.out | 58 +++++++++++++++++++++++++++++
src/pl/plpython/sql/plpython_spi.sql | 22 +++++++++++
3 files changed, 86 insertions(+), 1 deletions(-)