injection_points: Improve comment about disabled isolation permutation

9f00edc has disabled a permutation due to failures in the CI for
FreeBSD environments, but this is a matter of timing.  Let's document
properly why this type of permutation is a bad idea if relying on a wait
done in a SQL function, so as this can be avoided when implementing new
tests (this spec is also a template).

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/ZyCa2qsopKaw3W3K@paquier.xyz

Author: michaelpq

src/test/modules/injection_points/specs/basic.spec

@@ -26,7 +26,10 @@ step wakeup2	{ SELECT injection_points_wakeup('injection-points-wait'); }
 step detach2	{ SELECT injection_points_detach('injection-points-wait'); }
 
 # Detach after wait and wakeup.
-# This permutation is proving to be unstable on FreeBSD, so disable for now.
+# Permutations like the following one commented out should be avoided, as
+# the detach may finish before the SQL function doing the wait returns
+# its result.  It is recommended to use wakeups as the last permutation
+# should a wait be done within an SQL function.
 #permutation wait1 wakeup2 detach2
 
 # Detach before wakeup.  s1 waits until wakeup, ignores the detach.