Update mirroring instructions when adding a new mirror
### Problem
See thread: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/173877#note_2269796670
> In addition, it was not clear to me where to set the username, since the description in the UI **specifically** specifies to use the username in the URL when using SSH. But also **specifically** specifies to avoid username in the URL when using HTTPS. See quotes:
> `Do not include the username in the URL, use the username field below if required: https://gitlab.company.com/group/project.git.`
> `When using the ssh:// protocol, please use the following format: ssh://username@example.com/group/project.git.`
The guidance about using usernames in the URL contradicts to itself.
### Proposal
Review the list of instructions on add new mirror page and update them if necessary

issue