Show warning message for "merge immediately" on /merge via markdown

Problem to solve

In #12679 (closed) we introduced a warning when users select the "merge immediately" button. Unfortunately, no warning is shown when using markdown

Intended users

• Sasha (Software Developer)

Further details

Proposal

In case merge request is being added in markdown and merge request is enabled

Add the following helper text: Merging immediately isn't recommended as it may cause a relevant negative impact on the existing merge train.

Technical details

Add "Merge immediately (not recommended as it can cause a negative impact on an existing merge train). to the desc section of command :merge in lib/gitlab/quick_actions/merge_request_actions.rb

Ensure that this message only shows when the merge trains is enabled.

Permissions and Security

Documentation

Availability & Testing

What does success look like, and how can we measure that?

What is the type of buyer?

Links / references