docs: replace 'master' with 'root' where appropriate.
These uses of 'master' refer to partitioning / inheritance. 'root'
seems more descriptive than 'master'.
Author: Andres Freund
Reviewed-By: David Steele
Discussion: https://postgr.es/m/
20200615182235.x7lch5n6kcjq4aue@alap3.anarazel.de