pgsql: Improve documentation around parameter-setting and ALTER SYSTEM.

Improve documentation around parameter-setting and ALTER SYSTEM.

The ALTER SYSTEM ref page hadn't been held to a very high standard, nor
was the feature well integrated into section 18.1 (parameter setting).
Also, though commit 4c4654afe had improved the structure of 18.1, it also
introduced a lot of poor wording, imprecision, and outright falsehoods.
Try to clean that up.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/af06aa822dbc023f3a103278ec381e1c88d67db1

Modified Files
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doc/src/sgml/config.sgml           |  354 +++++++++++++++++++-----------------
doc/src/sgml/ref/alter_system.sgml |   55 +++---
2 files changed, 225 insertions(+), 184 deletions(-)