docs: add synopsis to set, update, list, and export
What does this MR do and why?
Adds Long (Synopsis) fields to four variable subcommands that were
missing them, as part of the style guide audit identified in #8263 (closed).
Commands updated:
variable setvariable updatevariable listvariable export
Content follows the documentation style guide and the GitLab CLI (glab) documentation style guide.