Doc: improve libpq service-file docs, avoid overspecifying pathnames.

Clarify libpq.sgml's description of service file locations and
semantics.  Avoid use of backtick'ed pg_config calls to describe
paths; that doesn't work on Windows, and even on Unix it's an
idiom that not all readers may be instantly familiar with.

Don't overspecify the locations of include files, instead writing
only as much as you'd use in #include directives.  The previous text
in these places was incorrect for some installations, depending on
where "postgresql" is in the install path.

Our convention for referencing the user's home directory seems
to be "~", so change the one place that spelled it "$HOME".

install-windows.sgml follows the platform convention of spelling
file paths with "\", so change the one place that used "/".

Haiying Tang and Tom Lane

Discussion: https://postgr.es/m/162149020918.26174.7150424047314144297@wrigleys.postgresql.org

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