docs(mr): add synopsis to approve, approvers, diff, close, delete, for,...
Description
This change adds helpful descriptions to various merge request commands in a GitLab CLI tool. Previously, these commands only had brief one-line descriptions, but now they include longer explanations that tell users what each command does and how to use it effectively.
The updates cover commands for approving, closing, deleting, viewing differences, creating, commenting on, reopening, and managing subscriptions for merge requests. Each description explains the command's purpose in plain language and mentions useful details like default behaviors (such as using the current branch when no specific merge request is specified) and available options.
This improvement makes the CLI tool more user-friendly by providing clearer guidance on what each command accomplishes, helping users understand their options without needing to consult external documentation.
Content follows the documentation style guide and the GitLab CLI (glab) documentation style guide.