docs(mr): add synopsis to list, view, create, checkout, merge, and update
Description
This change adds helpful documentation to six merge request commands in a GitLab CLI tool. Previously, these commands (checkout, create, list, merge, update, and view) had empty help text, but now they include brief descriptions explaining their default behavior and highlighting key options.
For example, the checkout command now explains it defaults to the current branch and mentions the --branch option, while the create command describes how --fill can auto-populate details from commit history. This makes the tool more user-friendly by providing context about what each command does and what options are available.
Content follows the documentation style guide and the GitLab CLI (glab) documentation style guide.
Related issues and merge requests
Edited by Brendan Lynch