Improve clarity in merge train error messaging documentation

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Overview

This issue addresses a minor wording improvement in the merge trains documentation to enhance clarity when explaining why merge requests are dropped from the train. The change replaces "For example, this could be caused by:" with "Common causes include:" to provide more direct and professional language that better communicates the nature of the listed items as typical causes rather than just examples.

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What needs to be done

• Change "For example, this could be caused by:" to "Common causes include:" in the merge request dropped section
• Ensure the wording flows naturally with the bullet points that follow
• Verify the change maintains technical accuracy
• Confirm consistency with documentation style standards

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Files to be changed

• doc/ci/pipelines/merge_trains.md

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Labels

~"group::documentation" ~"technical writing" ~"size::1"