Capacity based balancing was introduced in 2025.1. It computes balance
based on a node's capacity: the number of tablets located on a node
should be directly proportional to that node's storage capacity.
This change adds this explanation to the docs.
Fixes: #25686
Closes scylladb/scylladb#25687
(cherry picked from commit de5dab8429)
Closes scylladb/scylladb#26105