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
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/0fdadfb0192935e0f8bec7fa5ec9dbb470d6d587

Modified Files
--------------
doc/src/sgml/config.sgml | 89 +++++++-------
doc/src/sgml/jit.sgml | 169 ++++++++++++--------------
doc/src/sgml/release-11.sgml | 12 +-
src/backend/jit/jit.c | 4 +-
src/backend/utils/misc/guc.c | 4 +-
src/backend/utils/misc/postgresql.conf.sample | 15 +--
6 files changed, 145 insertions(+), 148 deletions(-)