ci(travis): improve recommended matrix usage comment

.travis.yml

@@ -13,12 +13,17 @@ services:
 
 # Make sure the instances listed below match up with
 # the `platforms` defined in `kitchen.yml`
-# NOTE: To minimise disruption across the whole org, only select five instances
-#       to run and leave the rest commented out
-#       The recommended matrix is shown below
-#       Allow `kitchen.yml` to contain all of the platforms, to allow
-#       comprehensive local testing
+# NOTE: Please try to select up to six instances that add some meaningful
+#       testing of the formula's behaviour. If possible, try to refrain from
+#       the classical "chosing all the instances because I want to test on
+#       another/all distro/s" trap: it will just add time to the testing (see
+#       the discussion on #121).  As an example, the set chosen below covers
+#       the most used distros families, systemd and non-systemd and the latest
+#       three supported Saltstack versions with python2 and 3."
+#       As for `kitchen.yml`, that should still contain all of the platforms,
+#       to allow for comprehensive local testing
 #       Ref: https://github.com/saltstack-formulas/template-formula/issues/118
+#       Ref: https://github.com/saltstack-formulas/template-formula/issues/121
 env:
   matrix:
     - INSTANCE: default-debian-9-2019-2-py3