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