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