docs(issuable): add synopsis to close, note, reopen, subscribe, and unsubscribe
Description
This change adds helpful descriptions to several GitLab CLI commands that previously had empty help text. The updates explain key features like being able to use full GitLab URLs instead of just ID numbers, that the note command opens an editor when no message is provided, and clarify what subscribing and unsubscribing to issues actually does (receiving or stopping notifications). These improvements make the command-line tool more user-friendly by providing clear guidance on how each command works.
Content follows the documentation style guide and the GitLab CLI (glab) documentation style guide.
Related issues and merge requests
Edited by Brendan Lynch