docs(variable): add synopsis and examples to variable commands
Description
Adds Long (Synopsis) fields to the variable command and its delete
and get subcommands, as part of the style guide audit identified in
#8263 (closed).
Each Synopsis explains the command's default behavior and key flags — how
variables relate to CI/CD pipelines, and when to use --scope and
--group. The Synopsis text is additive and does not repeat the Short
description.
Commands updated:
variablevariable deletevariable get
Content follows the documentation style guide and the GitLab CLI (glab) documentation style guide.
Related issues and merge requests
Documentation
If this MR adds or updates CLI command documentation, follow the conventions in the
GitLab CLI (glab) documentation style guide,
run make gen-docs, and commit the updated files in docs/source/.