doc: spell out full productname

Use PostgreSQL consistently for referring to the productname rather
than Postgres.  This also adds <productname> markup.

Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: "Jonathan S. Katz" <jkatz@postgresql.org>
Discussion: https://postgr.es/m/9C019644-9EA4-4B79-A52C-5C47A5B6B2DF@yesql.se

doc/src/sgml/pgbuffercache.sgml

@@ -1,7 +1,8 @@
 <!-- doc/src/sgml/pgbuffercache.sgml -->
 
 <sect1 id="pgbuffercache" xreflabel="pg_buffercache">
- <title>pg_buffercache &mdash; inspect Postgres buffer cache state</title>
+ <title>pg_buffercache &mdash; inspect <productname>PostgreSQL</productname>
+    buffer cache state</title>
 
  <indexterm zone="pgbuffercache">
   <primary>pg_buffercache</primary>

doc/src/sgml/postgres-fdw.sgml

@@ -2,7 +2,8 @@
 
 <sect1 id="postgres-fdw" xreflabel="postgres_fdw">
  <title>postgres_fdw &mdash;
-   access data stored in external Postgres servers</title>
+   access data stored in external <productname>PostgreSQL</productname>
+   servers</title>
 
  <indexterm zone="postgres-fdw">
   <primary>postgres_fdw</primary>