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

doc/src/sgml/filelist.sgml

@@ -166,6 +166,7 @@
 <!ENTITY sourcerepo SYSTEM "sourcerepo.sgml">
 
 <!ENTITY release    SYSTEM "release.sgml">
+<!ENTITY release-12     SYSTEM "release-12.sgml">
 <!ENTITY release-11     SYSTEM "release-11.sgml">
 <!ENTITY release-10     SYSTEM "release-10.sgml">
 <!ENTITY release-9.6    SYSTEM "release-9.6.sgml">

doc/src/sgml/release-12.sgml

+<!-- doc/src/sgml/release-12.sgml -->
+<!-- See header comment in release.sgml about typical markup -->
+
+ <sect1 id="release-12">
+  <title>Release 12</title>
+
+  <para>JIT is enabled by default in this release.  It was disabled by
+  default in PG 11, so we document is enablement here.
+  </para>
+
+ </sect1>

doc/src/sgml/release.sgml

@@ -76,6 +76,7 @@ For new features, add links to the documentation sections.
   The reason for splitting the release notes this way is so that appropriate
   subsets can easily be copied into back branches.
 -->
+&release-12;
 &release-11;
 &release-10;
 &release-9.6;