docs: Clarify time format for glab token rotate

Description

Feedback:

Hi team, one of my customers reached out being confused about the token duration documented here. The CLI docs say 'the token expires at midnight UTC on the calculated date,' which is technically accurate but can IMO be misleading. 'Midnight on a date' is ambiguous: it could mean the start (00:00) or end (24:00) of that day. And we use the start of the day (00:00 UTC), not the end. The customer understood that if they created a token with a one-day expiry, the calculated date would be tomorrow, and that midnight on that day would be at 24:00, not 00:00. However, tokens with a one-day expiry created a few minutes ago show that they will expire in fourteen hours.

Request:

Would it make sense to clarify our meaning of 'midnight UTC' in this context?

Changes

This merge request improves the documentation and help text for the token rotation command. The changes make the explanations clearer and more concise by simplifying the description of how token expiration works, clarifying that tokens expire at midnight UTC (00:00) on the calculated date

The functionality remains exactly the same. This is purely about making the docs and user-facing help text more user-friendly and easier to follow.

Related issues and merge requests

• Slack thread

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/.