More informative descriptions for parametrized jobs

In particular, highlight the fact that some jobs run (almost) nothing
by default.

Change-Id: Ibd6a52f5d73dc45e72bf26bf10a740a1def9e1b3
diff --git a/mbedtls-release-ci-testing.yaml b/mbedtls-release-ci-testing.yaml
index 734e1fb..2a18123 100644
--- a/mbedtls-release-ci-testing.yaml
+++ b/mbedtls-release-ci-testing.yaml
@@ -1,6 +1,12 @@
 - job:
     concurrent: true
-    description: Mbed TLS Release job (new)
+    description: |
+      Mbed TLS release job on a chosen branch of <tt>mbedtls-test</tt>.
+      <p>
+      Check at least one of the <tt>RUN_</tt> parameters!
+      Otherwise not much will happen.
+      <p>
+      Choose the branch of <tt>mbedtls-test</tt> under <tt>TEST_BRANCH</tt>.
     disabled: false
     name: mbedtls-release-ci-testing
     parameters: