docs: add synopsis to experimental subcommands
What does this MR do and why?
Adds Long (Synopsis) fields to the nine runner-controller experimental
subcommands that were missing them, as part of the style guide audit
identified in #8263.
Synopsis fields are added only where there is genuine content beyond what
the Short field already communicates.
Subcommands updated:
runner-controller createrunner-controller deleterunner-controller listrunner-controller updaterunner-controller scope listrunner-controller token createrunner-controller token listrunner-controller token revokerunner-controller token rotate
Content follows the documentation style guide and the GitLab CLI (glab) documentation style guide.