This clarifies some wording in the description of the options available
as replication solutions.  While on it, this replaces some instances of
"master" with "primary", for consistency with recent changes like
9e101cf6.

Author: Robert Treat
Reviewed-by: Magnus Hagander, Michael Paquier
Discussion: https://postgr.es/m/CAJSLCQ2TPaK_K8raofCamrqELCxY-H6mJrpDNRzc-LKpPY7c+g@mail.gmail.com