Document Praefect storage node constraints

What does this MR do?

Updates documentation for Gitaly Cluster with regard to virtual storage Gitaly node constraints.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

Low risk, only a documentation update.