[docs] Update information about restricting jobs in paralell
This is Slack chat with @otinto that also confirmed with @dbeltran;
We have this page with information about how to control the number of jobs waiting or running in parallel for an experiment: https://autosubmit.readthedocs.io/en/master/userguide/configure/develop_a_project.html#controlling-the-number-of-active-concurrent-tasks-in-an-experiment
Pre 4.1, users had to duplicate the platform to have a second identical platform but with a different limits for jobs.
From 4.1 onward, the JOB can have the same settings from the platform (right @dbeltran ?).
We need to add that to this page, check if the Variables page also needs to be updated, and add a note for pre 4.1 users.