docs: Add caveats for Windows as the build platform

Commit cf60ccc ("cutils: Introduce bundle mechanism") introduced
a Python script to populate a bundle directory using os.symlink() to
point to the binaries in the pc-bios directory of the source tree.
Commit 882084a ("datadir: Use bundle mechanism") removed previous
logic in pc-bios/meson.build to create a link/copy of pc-bios binaries
in the build tree so os.symlink() is the way to go.

However os.symlink() may fail [1] on Windows if an unprivileged Windows
user started the QEMU build process, which results in QEMU executables
generated in the build tree not able to load the default BIOS/firmware
images due to symbolic links not present in the bundle directory.

This commits updates the documentation by adding such caveats for users
who want to build QEMU on the Windows platform.

[1] https://docs.python.org/3/library/os.html#os.symlink

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Message-Id: <20220719135014.764981-1-bmeng.cn@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/about/build-platforms.rst

@@ -94,8 +94,16 @@ not tested anymore, so it is recommended to use one of the latest versions of
 Windows instead.
 
 The project supports building QEMU with current versions of the MinGW
-toolchain, either hosted on Linux (Debian/Fedora) or via MSYS2 on Windows.
+toolchain, either hosted on Linux (Debian/Fedora) or via `MSYS2`_ on Windows.
+A more recent Windows version is always preferred as it is less likely to have
+problems with building via MSYS2. The building process of QEMU involves some
+Python scripts that call os.symlink() which needs special attention for the
+build process to successfully complete. On newer versions of Windows 10,
+unprivileged accounts can create symlinks if Developer Mode is enabled.
+When Developer Mode is not available/enabled, the SeCreateSymbolicLinkPrivilege
+privilege is required, or the process must be run as an administrator.
 
 .. _Homebrew: https://brew.sh/
 .. _MacPorts: https://www.macports.org/
+.. _MSYS2: https://www.msys2.org/
 .. _Repology: https://repology.org/