Improve messaging for adding to a merge train when merge request pipeline is in progress

🧩 Problem statement

In #283899 (closed) we researched the usefulness of the text presented in the widget for adding an MR to a train when a merge request pipeline is in progress.

The common interpretation of the message is When the pipeline in progress succeeds, the merge train will Start/MR will be added to a train which is very similar to what's been communicated through the button text. For first time users, the reason for waiting on the success of the pipeline to add the MR to a train is not clarified until they go to the documentation.

📝 Proposal

• Reframe the message shown in the widget to match the documentation
• Use a dismissable banner to present the message to onboarding users