Fix issues around EXPLAIN with JIT.

I (Andres) was more than a bit hasty in committing 33001fd7a7
after last minute changes, leading to a number of problems (jit output
was only shown for JIT in parallel workers, and just EXPLAIN without
ANALYZE didn't work).  Lukas luckily found these issues quickly.

Instead of combining instrumentation in in standard_ExecutorEnd(), do
so on demand in the new ExplainPrintJITSummary().

Also update a documentation example of the JIT output, changed in
52050ad8eb.

Author: Lukas Fittl, with minor changes by me
Discussion: https://postgr.es/m/CAP53PkxmgJht69pabxBXJBM+0oc6kf3KHMborLP7H2ouJ0CCtQ@mail.gmail.com
Backpatch: 11, where JIT compilation was introduced

doc/src/sgml/jit.sgml

@@ -169,12 +169,8 @@ SET
  Planning Time: 0.133 ms
  JIT:
    Functions: 3
-   Generation Time: 1.259 ms
-   Inlining: false
-   Inlining Time: 0.000 ms
-   Optimization: false
-   Optimization Time: 0.797 ms
-   Emission Time: 5.048 ms
+   Options: Inlining false, Optimization false, Expressions true, Deforming true
+   Timing: Generation 1.259 ms, Inlining 0.000 ms, Optimization 0.797 ms, Emission 5.048 ms, Total 7.104 ms
  Execution Time: 7.416 ms
 </screen>
    As visible here, <acronym>JIT</acronym> was used, but inlining and