Backup Preparations based on head backups and need to be updated/clarified.

Description

The documentation talking about 2TB limit for uncompressed is inaccurate.  With the new headless backup model, i.e. FCV 4.2+, we are no longer reading the data directly, but pulling the blocks directly from the target node.  That means for the newer backup model, the data is compressed and therefore should be based on filesize and not data size. 

https://docs.opsmanager.mongodb.com/current/core/backup-preparations/#backup-sizing-recommendation 

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)