doc: replace DOS with MSYS shell to be more consistent
doc: replace DOS with MSYS shell to be more consistent

A plain DOS window doesn't have the right path settings, whereas
the MSYS shell window does.

Conflicts:
    doc/build-msw.md

Rebased-from: 51e3e0909eb8567cde3fe1be0badbc266cb4cc6a 0.8.x

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