pgsql: doc: JIT is enabled by default in PG 12

doc:  JIT is enabled by default in PG 12

JIT was disabled by default in a PG 11 in a separate commit that will
normally not appear in the PG 12 git logs.  Therefore, create a PG 12
document and mention the fact that JIT is enabled by default in this
release.  (A similar change in parallelism was missed in a prior
release.)

Reported-by: Andres Freund

Discussion: https://postgr.es/m/20180922000554.qukbhhlagpnopvko@alap3.anarazel.de

Backpatch-through: head

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/1f7fc7670c7e0051b47bb4a4d6b41ff9e8af53fa

Modified Files
--------------
doc/src/sgml/filelist.sgml   |  1 +
doc/src/sgml/release-12.sgml | 11 +++++++++++
doc/src/sgml/release.sgml    |  1 +
3 files changed, 13 insertions(+)