Clarify what reset-data does to avoid ambiguity about what git repository is reset

What does this merge request do and why?

When trying to reset all my GitLab data, I found the gdk reset-data command. However, I decided not to use it because I thought "Back up and create fresh git repository" meant that it would reset my local gitlab/ directory and I'd lose any local branches etc.

In reality what reset-data does is simply reset the Gitaly storage.

To prevent anyone else making the same mistake, I've attempted to clarify what the command does, and updated the docs with some more details provided by @kkloss (Thank you!)

How to set up and validate locally

1. Run gdk help and check the help for reset-data

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.