45463: test: kill: Document why we use SIGURG

See discussion in workers/45460.

ChangeLog

@@ -1,5 +1,8 @@
 2020-02-18  Chris Down  <chris@chrisdown.name>
 
+	* 45463: Test/B11kill.ztst: test: kill: Document why we use
+	SIGURG
+
 	* 45453: Src/jobs.c, Test/B11kill.ztst: builtins: kill: Do not
 	signal current process group when pid is empty
 

Test/B11kill.ztst

@@ -60,6 +60,15 @@
 ?(eval):kill:2: not enough arguments
 
 # Regression tests: `kill ''` should not result in `kill 0`.
+#
+# We use SIGURG where an explicit sigspec can be provided as:
+#
+# 1. By default it's non-terminal, so even if we regress, we won't kill the
+#    test runner and other processes in the process group since we'll stop
+#    running this test before we get to the plain kill (and thus SIGTERM)
+#    cases;
+# 2. It's also unlikely to be sent for any other reason during the process
+#    lifetime, so the test shouldn't be flaky.
 
   (
     trap 'exit 11' URG