pgsql: Add documentation for the JIT feature.

Add documentation for the JIT feature.

As promised in earlier commits, this adds documentation about the new
build options, the new GUCs, about the planner logic when JIT is used,
and the benefits of JIT in general.

Also adds a more implementation oriented README.

I'm sure we're going to want to expand this further, but I think this
is a reasonable start.

Author: Andres Freund, with contributions by Thomas Munro
Reviewed-By: Thomas Munro
Discussion: https://postgr.es/m/20170901064131.tazjxwus3k2w3ybh@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e6c039d13e16a3a2dec5ba479d9d1fb3229c03a3

Modified Files
--------------
doc/src/sgml/acronyms.sgml     |  10 ++
doc/src/sgml/config.sgml       | 183 ++++++++++++++++++++++++-
doc/src/sgml/filelist.sgml     |   1 +
doc/src/sgml/func.sgml         |   8 ++
doc/src/sgml/installation.sgml |  53 ++++++++
doc/src/sgml/jit.sgml          | 299 +++++++++++++++++++++++++++++++++++++++++
doc/src/sgml/postgres.sgml     |   1 +
doc/src/sgml/storage.sgml      |   2 +-
src/backend/jit/README         | 289 +++++++++++++++++++++++++++++++++++++++
9 files changed, 844 insertions(+), 2 deletions(-)