Update project settings copy for merge trains support for fast forward merge

Problem

When fast forward and merge train are enabled together, when the master branch advances, the only way users could merge their MR is by manually rebasing their feature branch on the master and pushing it to the remote repository.

More details about this issue could be found in #35628 (closed)

Proposal

General: When a fast-forward merge with merge train is not possible, the application will try to rebase - this will rebase the entire merge train. Rebasing on merge train should happen automatically and shouldn't require a manual action from users.

In this issue, update the Project settings > General > Merge request section with the following changes:

• Add a new line of copy to the Fast-forward merge option explaining if it activated together with merge trains, merge is only allowed if the branch can be fast-forwarded
  • Once merge trains are enabled, merging is only allowed if the branch can be fast forwarded
• Add a link to the documentation page: NEW SECTION ON DOCS, NEEDS TO BE CREATED cc @marcia
• Update the copy for merge pipelines and add link to merge trains documentation
  • Enables merge train by default. Merge pipelines will try to validate the post-merge result prior to merging
• Add a link to the documentation page: https://docs.gitlab.com/ee/ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/

Reference the old designs created for the proposal here.