SSH docs: clarify that you need to accept the authenticity of the server

What does this MR do?

Clarify that you need to accept the authenticity of the server when connecting to GitLab with SSH.

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54880

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards