docs: clarify glab token rotate command info
Description
The following user feedback was provided:
Any hint what "rotate" in this context means would be nice. It is somewhat implicit in the
-DFlag, but an explicit statement would be helpful.
The changes add a brief explanation to the rotate command's documentation, clarifying what actually happens when a token is rotated: the old token is immediately invalidated and replaced with a new one that has a fresh secret value and expiration date.
Users are also reminded that any applications or services using the old token must be updated with the new value right away. This same explanation is added in two places — the user-facing documentation file and the command's built-in help text — so the information is consistent wherever users look for guidance.
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/.