Doc: tweak installation doc edits made by commit
76c2af926.
We don't consider that building with MinGW is deprecated,
so adjust some places that gave that impression.
Per discussion with Peter Eisentraut.
Discussion: https://postgr.es/m/
4a023388-8652-fea0-a0b4-
35ad5e734e9a@2ndquadrant.com