docs(snippet): add synopsis and examples to snippet commands
Description
Adds Long (Synopsis) fields to the snippet command and its create
subcommand, as part of the style guide audit identified in #8263 (closed).
Each Synopsis explains what snippets are and the command's default
behavior — uploading file paths versus piping from standard input, and
when to use --personal and --visibility. The Synopsis text is additive
and does not repeat the Short description.
Commands updated:
snippetsnippet create
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/.