Show description of 'gettext:regenerate' task

What does this MR do?

rake gettext:regenerate is used to regenerate the gitlab.pot file, particularly when there are merge conflicts. Make it easier to find by showing it in rake -T

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content