doc: Document how to use privileged port for sshd

What does this Merge Request do and why?

When you want to GDK to run sshd on for example port 22 it will need extra capabilities.

This change adds instructions in to the documentation how to do this.

Change originated from this comment.

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 Issue to follow-up.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.
• [-] Add the ~highlight label if this MR should be included in the CHANGELOG.md.