Clarify docs regarding merged results pipelines
Problem to solve
From the discussion at #212875 (comment 314080258), I think we need a little more tweaking to the documentation for pipelines for merged results and merge trains.
We should clarify that pipelines for merge requests is the CI configuration that is needed, and that enabling this feature affects those merge request pipelines specifically.
Other links/references
Related to #212875 (comment 314080258)
Related to !27255 (merged)
Edited by Marcel Amirault