pgsql: In v11,disable JIT by default (it's still enabled by default in

In v11, disable JIT by default (it's still enabled by default in HEAD).

Per discussion, JIT isn't quite mature enough to ship enabled-by-default.

I failed to resist the temptation to do a bunch of copy-editing on the
related documentation.  Also, clean up some inconsistencies in which
section of config.sgml the JIT GUCs are documented in vs. what guc.c
and postgresql.config.sample had.

Discussion: https://postgr.es/m/20180914222657.mw25esrzbcnu6qlu@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8f32bacc006692e63f137924d5849b3cb9f2d4d3

Modified Files
--------------
doc/src/sgml/config.sgml                      |  87 +++++++------
doc/src/sgml/jit.sgml                         | 169 ++++++++++++--------------
doc/src/sgml/release-11.sgml                  |  12 +-
src/backend/jit/jit.c                         |   2 +-
src/backend/utils/misc/guc.c                  |   2 +-
src/backend/utils/misc/postgresql.conf.sample |  15 +--
6 files changed, 142 insertions(+), 145 deletions(-)