docs(iteration): add synopsis and examples to iteration commands
Description
Adds Long (Synopsis) fields to the iteration command and its list
subcommand, as part of the style guide audit identified in #8263 (closed).
Each Synopsis explains what iterations are, the command's default
behavior, and when to use --group to target a group or --repo to
target another project. The Synopsis text is additive and does not repeat
the Short description.
Commands updated:
iterationiteration list
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/.