Include the factor/range to determine Confluence instance size in Database Migration Documentation.

In our database migration documentation, we have a section saying that:

The XML export built into Confluence is not suited for the backup or migration of large data sets.

However, we did not provide customer to the factor that determine the size of the "large data" like JIRA have is documented below:

• JIRA Sizing Guide

Therefore, it is useful to include the information for Confluence specifically so that we are aware that XML backup is not a production friendly for specific instance size.