Update mirroring instructions when adding a new mirror

Problem

See thread: !173877 (comment 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