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