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

dafab2b 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 dafab2b
  hebasto:
    ACK dafab2b

Tree-SHA512: d8d50fa13c34be753700843183a1a4addad6447c813bfc039865ba11d5fa03c25206ed6f3782331df4c6a44448e3fec4138ae911b995b5cacc5b756bb5d74bb4

Author: laanwj

doc/build-windows.md

@@ -81,9 +81,26 @@ The first step is to install the mingw-w64 cross-compilation tool chain:
 
     sudo apt install g++-mingw-w64-x86-64
 
-Ubuntu Bionic 18.04 <sup>[1](#footnote1)</sup>:
+Next, set the default `mingw32 g++` compiler option to POSIX<sup>[1](#footnote1)</sup>:
 
-    sudo update-alternatives --config x86_64-w64-mingw32-g++ # Set the default mingw32 g++ compiler option to posix.
+```
+sudo update-alternatives --config x86_64-w64-mingw32-g++
+```
+
+After running the above command, you should see output similar to that below.
+Choose the option that ends with `posix`.
+
+```
+There are 2 choices for the alternative x86_64-w64-mingw32-g++ (providing /usr/bin/x86_64-w64-mingw32-g++).
+
+  Selection    Path                                   Priority   Status
+------------------------------------------------------------
+  0            /usr/bin/x86_64-w64-mingw32-g++-win32   60        auto mode
+* 1            /usr/bin/x86_64-w64-mingw32-g++-posix   30        manual mode
+  2            /usr/bin/x86_64-w64-mingw32-g++-win32   60        manual mode
+
+Press <enter> to keep the current choice[*], or type selection number:
+```
 
 Once the toolchain is installed the build steps are common: