pgsql: pgbench: Change terminology from "threshold" to "parameter".

pgbench: Change terminology from "threshold" to "parameter".

Per a recommendation from Tomas Vondra, it's more helpful to refer to
the value that determines how skewed a Gaussian or exponential
distribution is as a parameter rather than a threshold.

Since it's not quite too late to get this right in 9.5, where it was
introduced, back-patch this. Most of the patch changes only comments
and documentation, but a few pgbench messages are altered to match.

Fabien Coelho, reviewed by Michael Paquier and by me.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/2c5b57ec7f0155eef7bd8d152e546ee96b2feb5e

Modified Files
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doc/src/sgml/ref/pgbench.sgml | 67 +++++++++++++++++++++-----------------
src/bin/pgbench/pgbench.c | 71 +++++++++++++++++++++++------------------
2 files changed, 78 insertions(+), 60 deletions(-)