docs: improve glab changelog and glab check-update command docs
Description
This merge request improves the documentation and help text for the glab changelog and glab check-update commands.
The changes make the command descriptions clearer and more detailed. The updates also provide better examples showing different ways to use the command (like generating changelogs for specific versions or commit ranges) and clarify the command-line options with more precise descriptions.
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/.
Edited by Brendan Lynch