Document the backup-timeout option to gitlab-ctl replicate-geo-database

What does this MR do?

Adds documentation for the --backup-timeout option to gitlab-ctl replicate-geo-database.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Without this, Geo sysadmins with large postgresql databases are flummoxed.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Has been reviewed by Documentation
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes omnibus-gitlab#2998 (closed)