Doc: improve documentation about where the psqlrc files are.

Doc: improve documentation about where the psqlrc files are.

Remove no-longer-accurate claim that Windows lacks home directories.
Clarify the text by more clearly distinguishing which statements
reflect hard-wired choices versus which ones reflect overridable
defaults.  Update the examples of version-specific file names,
and make them track future version changes by using "&majorversion;"
and "&version;".  (BTW, in devel and beta releases this method
correctly says that you can use strings like "16devel" and "15beta4"
as minor version identifiers.)

Back-patch to v15, but not further, with the thought that in older
releases the examples with three-part version numbers still had
some historical relevance.  v15 will be the first major release after
the last 9.x branch went out of support.

Robert Treat and Tom Lane, reviewed by Julien Rouhaud

Discussion: https://postgr.es/m/CAJSLCQ07F-WCYYYOY8+dWhHcVeJ1Pb01cWc-c0Hu=M3EjKT2Eg@mail.gmail.com