docs: ensure glab check-update is accurately described
Description
Instead of saying "upgrade glab" or "check for latest glab releases," the text now says "check for the latest glab version." This makes the language clearer because the command doesn't actually perform an upgrade. It only checks what version is available. The changes appear in help text, documentation, and user-facing messages to ensure consistent terminology across the application.
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/.