Document a failure mode for large repositories in Geo

What does this MR do?

Adds a caveat about Geo + large repositories to the troubleshooting document

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

Why was this MR needed?

We didn't manage to increase the default timeout in time for %10.2

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
  • Has been reviewed by Backend
• 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?

Related to gitlab-ce#39977