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

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