Merge bitcoin/bitcoin#22088: doc: improve note on choosing posix mingw32

dafab2b1b37d1966610b2189e71c52e3af38dfaa doc: improve note on choosing posix mingw32 (Jarol Rodriguez)

Pull request description:

  The current [windows build doc](https://github.com/bitcoin/bitcoin/blob/master/doc/build-windows.md) can lead someone to believe that the step where you must choose the posix mingw32 g++ compiler option is only for `Ubuntu 18.04`. It is only until you (or just me) go through the cross-compile process and realize that it's not building because you didn't set this option on > `Ubuntu 18.04`. Then you come back and read the footnotes and see: `Starting from Ubuntu Xenial 16.04...`.

  This PR improves this portion of the doc by editing the text around this stage to state "this is now the next step". We could add a note saying `Ubuntu 18.04 and up`, but this is redundant as it's unlikely someone will be using < Ubuntu 18.04 since it's not officially supported by our build system. While here, some minor fixups and add some more guidance to completing this step.

  **Master:** [render](https://github.com/bitcoin/bitcoin/blob/master/doc/build-windows.md#building-for-64-bit-windows)
  **PR:** [render](https://github.com/bitcoin/bitcoin/blob/dafab2b1b37d1966610b2189e71c52e3af38dfaa/doc/build-windows.md#building-for-64-bit-windows)

ACKs for top commit:
  laanwj:
    ACK dafab2b1b37d1966610b2189e71c52e3af38dfaa
  hebasto:
    ACK dafab2b1b37d1966610b2189e71c52e3af38dfaa

Tree-SHA512: d8d50fa13c34be753700843183a1a4addad6447c813bfc039865ba11d5fa03c25206ed6f3782331df4c6a44448e3fec4138ae911b995b5cacc5b756bb5d74bb4

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