PR welcome msg: update the msg to cover additional Packit cmds

I found useful to be able to run just failed tests (when they crashed
due to infrastructure issues). Updating the welcome msg to provide
this info and little bit restructured original statement, to make
it more friendly for reading.

.github/workflows/pr-welcome-msg.yml

@@ -21,16 +21,21 @@ jobs:
             ## **Thank you for contributing to the Leapp project!**
             Please note that every PR needs to comply with the [Leapp Guidelines](https://leapp.readthedocs.io/en/latest/contributing.html#) and must pass all tests in order to be mergeable.
             If you want to request a review or rebuild a package in copr, you can use following commands as a comment:
-            - **review please @oamg/developers** to notify leapp developers of the review request
-            - **/packit copr-build** to submit a public copr build using packit
+            - **`review please @oamg/developers`** to notify leapp developers of the review request
+            - **`/packit copr-build`** to submit a public copr build using packit
 
-            Packit will automatically schedule regression tests for this PR's build and latest upstream leapp build. If you need a different version of leapp, e.g. from PR#42, use `/packit test oamg/leapp#42`
-            Note that first time contributors cannot run tests automatically - they will be started by a reviewer.
+            Packit will automatically schedule regression tests for this PR's build and latest upstream leapp build.
+            However, here are additional useful commands for packit:
+            - **`/packit test`** to re-run manually the default tests
+            - **`/packit retest-failed`** to re-run failed tests manually
+            - **`/packit test oamg/leapp#42`** to run tests with leapp builds for the leapp PR#42 (default is latest upstream - master - build)
+
+            Note that first time contributors cannot run tests automatically - they need to be started by a reviewer.
 
             It is possible to schedule specific on-demand tests as well. Currently 2 test sets are supported, `beaker-minimal` and `kernel-rt`, both can be used to be run on all upgrade paths or just a couple of specific ones.
             To launch on-demand tests with packit:
-            - **/packit test --labels kernel-rt** to schedule `kernel-rt` tests set for all upgrade paths
-            - **/packit test --labels beaker-minimal-8.10to9.4,kernel-rt-8.10to9.4** to schedule `kernel-rt` and `beaker-minimal` test sets for 8.10->9.4 upgrade path
+            - **`/packit test --labels kernel-rt`** to schedule `kernel-rt` tests set for all upgrade paths
+            - **`/packit test --labels beaker-minimal-8.10to9.4,kernel-rt-8.10to9.4`** to schedule `kernel-rt` and `beaker-minimal` test sets for 8.10->9.4 upgrade path
 
             See other labels for particular jobs defined in the `.packit.yaml` file.