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 ba356a39 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

@@ -7775,9 +7775,11 @@ 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
-      to <filename>~/.pg_service.conf</filename>
+      connection service file
       (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>
 
@@ -8081,7 +8083,7 @@ 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>
@@ -8150,8 +8152,12 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
    Service names can be defined in either a per-user service file or a
    system-wide file.  If the same service name exists in both the user
    and the system file, the user file takes precedence.
-   By default, the per-user service file is located
-   at <filename>~/.pg_service.conf</filename>; this can be overridden by
+   By default, the per-user service file is named
+   <filename>~/.pg_service.conf</filename>.
+   On Microsoft Windows, it is named
+   <filename>%APPDATA%\postgresql\.pg_service.conf</filename> (where
+   <filename>%APPDATA%</filename> refers to the Application Data subdirectory
+   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