Doc: clarify location of libpq's default service file on Windows.

The documentation didn't specify the name of the per-user service file
on Windows, and extrapolating from the pattern used for other config
files gave the wrong answer.  The fact that it isn't consistent with the
others sure seems like a bug, but it's far too late to change that now;
we'd just penalize people who worked it out in the past.  So, simply
document the true state of affairs.

In passing, fix some gratuitous differences between the discussions
of the service file and the password file.

Julien Rouhaud, per question from Dominique Devienne.

Backpatch to all supported branches.  I (tgl) also chose to back-patch
the part of commit ba356a397 that touched libpq.sgml's description of
the service file --- in hindsight, I'm not sure why I didn't do so at
the time, as it includes some fairly essential information.

Discussion: https://postgr.es/m/CAFCRh-_mdLrh8eYVzhRzu4c8bAFEBn=rwoHOmFJcQOTsCy5nig@mail.gmail.com

doc/src/sgml/libpq.sgml

@@ -7103,9 +7103,11 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
        <primary><envar>PGSERVICEFILE</envar></primary>
       </indexterm>
       <envar>PGSERVICEFILE</envar> specifies the name of the per-user
-      connection service file.  If not set, it defaults
+      connection service file
-      to <filename>~/.pg_service.conf</filename>
       (see <xref linkend="libpq-pgservice"/>).
+      Defaults to <filename>~/.pg_service.conf</filename>, or
+      <filename>%APPDATA%\postgresql\.pg_service.conf</filename> on
+      Microsoft Windows.
      </para>
     </listitem>
 
@@ -7389,7 +7391,7 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
    <filename>%APPDATA%\postgresql\pgpass.conf</filename> (where
    <filename>%APPDATA%</filename> refers to the Application Data subdirectory in
    the user's profile).
-   Alternatively, a password file can be specified
+   Alternatively, the password file to use can be specified
    using the connection parameter <xref linkend="libpq-connect-passfile"/>
    or the environment variable <envar>PGPASSFILE</envar>.
   </para>
@@ -7448,26 +7450,34 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
   <para>
    The connection service file allows libpq connection parameters to be
    associated with a single service name. That service name can then be
-   specified by a libpq connection, and the associated settings will be
+   specified in a libpq connection string, and the associated settings will be
    used. This allows connection parameters to be modified without requiring
-   a recompile of the libpq application. The service name can also be
+   a recompile of the libpq-using application. The service name can also be
    specified using the <envar>PGSERVICE</envar> environment variable.
   </para>
 
   <para>
-   The connection service file can be a per-user service file
+   Service names can be defined in either a per-user service file or a
-   at <filename>~/.pg_service.conf</filename> or the location
+   system-wide file.  If the same service name exists in both the user
-   specified by the environment variable <envar>PGSERVICEFILE</envar>,
+   and the system file, the user file takes precedence.
-   or it can be a system-wide file
+   By default, the per-user service file is named
-   at <filename>`pg_config --sysconfdir`/pg_service.conf</filename> or in the directory
+   <filename>~/.pg_service.conf</filename>.
-   specified by the environment variable
+   On Microsoft Windows, it is named
-   <envar>PGSYSCONFDIR</envar>.  If service definitions with the same
+   <filename>%APPDATA%\postgresql\.pg_service.conf</filename> (where
-   name exist in the user and the system file, the user file takes
+   <filename>%APPDATA%</filename> refers to the Application Data subdirectory
-   precedence.
+   in the user's profile).  A different file name can be specified by
+   setting the environment variable <envar>PGSERVICEFILE</envar>.
+   The system-wide file is named <filename>pg_service.conf</filename>.
+   By default it is sought in the <filename>etc</filename> directory
+   of the <productname>PostgreSQL</productname> installation
+   (use <literal>pg_config --sysconfdir</literal> to identify this
+   directory precisely).  Another directory, but not a different file
+   name, can be specified by setting the environment variable
+   <envar>PGSYSCONFDIR</envar>.
   </para>
 
   <para>
-   The file uses an <quote>INI file</quote> format where the section
+   Either service file uses an <quote>INI file</quote> format where the section
    name is the service name and the parameters are connection
    parameters; see <xref linkend="libpq-paramkeywords"/> for a list.  For
    example:
@@ -7478,9 +7488,22 @@ host=somehost
 port=5433
 user=admin
 </programlisting>
-   An example file is provided at
+   An example file is provided in
+   the <productname>PostgreSQL</productname> installation at
    <filename>share/pg_service.conf.sample</filename>.
   </para>
+
+  <para>
+   Connection parameters obtained from a service file are combined with
+   parameters obtained from other sources.  A service file setting
+   overrides the corresponding environment variable, and in turn can be
+   overridden by a value given directly in the connection string.
+   For example, using the above service file, a connection string
+   <literal>service=mydb port=5434</literal> will use
+   host <literal>somehost</literal>, port <literal>5434</literal>,
+   user <literal>admin</literal>, and other parameters as set by
+   environment variables or built-in defaults.
+  </para>
  </sect1>