Improve merge request pipelines configuration info

Doc https://docs.gitlab.com/ee/ci/pipelines/merge_request_pipelines.html

Issue From the documentation it is not neither clear or evident that just adding the property rules to a job fires a merge request pipeline when a commit is pushed on the source branch. Following what happened. I had my branch, say A, with an open merge request on the target branch B and I was playing with the jobs properties, but without ever using the rules property. Until here everything was clear, every time I pushed a commit only the branch pipeline started. The strange thing happened once I started to play with the rules property, after that every time I pushed a commit 2 pipelines started, the branch pipeline and the merge request pipeline. So from the documentation it is not very clear that without the rules property only the branch pipeline is fired, but as soon as the rules property is used branch and merge requests pipelines are fired at every commit (unless other conditions are set).