Further clarification of in-place on Windows.

This PR explicitly enumerates what in-place functionality is and isn't available under Windows. It aims to fix the issue discovered by @manics (5.0.3 testing spreadsheet, row 29).
Care will have to be taken when rebasing, as the ``.. note`` element won't be present on ``develop``.

To test: please make sure if the limitations are clear enough.

\cc @manics, @joshmoore, @pwalczysko, @mtbc, @rleigh-dundee, @sbesson
Minor nit, but it's `POSIX`.