pgsql: Improve SPI documentation about null-flags arrays.

Improve SPI documentation about null-flags arrays.

Clarify the description of nulls[] arguments, and use the same wording
for all SPI functions with this type of argument.  Per gripe from Yuriy
Rusinov.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/c091c431979c182bc835b345655c1c162479aeb3

Modified Files
--------------
doc/src/sgml/spi.sgml |  105 +++++++++++++++++++++++++++++++++----------------
1 files changed, 71 insertions(+), 34 deletions(-)