Add documentation about percentage speed limit and data limit estimation
Add documentation about percentage speed limit and data limit estimation

(cherry picked from commit 5eb2a6872542cb31bf3da23f27eeb13735c2e484)

uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend