Clarify setup for parallel job execution performance test

Explicitly state, that parallelism enhancing options are to be used
in parallel performance tests, if it is best practice to use them on
the respective versions. A prerequisite for those options is that
using them does not require, even under heavy load but assuming enough
capacity in the cluster, any jobs to be retried.
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>

doc/design-performance-tests.rst

@@ -87,8 +87,12 @@ number of instances and parallel jobs can be tested realistically.
 The following tests are added to the QA:
 
   * Submitting twice as many instance creation request as there are
-    nodes in the cluster, using DRBD as disk template. As soon as a
-    creation job succeeds, submit a removal job for this instance.
+    nodes in the cluster, using DRBD as disk template.
+    The job parameters are chosen according to best practice for
+    parallel instance creation without running the risk of instance
+    creation failing for too many parallel creation attempts.
+    As soon as a creation job succeeds, submit a removal job for
+    this instance.
   * Submitting twice as many instance creation request as there are
     nodes in the cluster, using Plain as disk template. As soon as a
     creation job succeeds, submit a removal job for this instance.