pgsql: doc: Improve some documentation about HA and replication

doc: Improve some documentation about HA and replication

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
9e101cf.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8550cbd0bae7c3004034351cb3447b51a552e56c

Modified Files
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doc/src/sgml/high-availability.sgml | 46 ++++++++++++++++++-------------------
1 file changed, 22 insertions(+), 24 deletions(-)