doc: Meson is not experimental on Windows

The installation documentation stated that using Meson is
experimental.  But since this is the only way to build using Visual
Studio on Windows, this would imply that that whole build procedure is
experimental, which isn't true.  So qualify this statement a bit more.
We keep the statement that Meson is experimental on other platforms,
since it doesn't have full, confirmed feature parity with the make
build system.

Author: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://www.postgresql.org/message-id/flat/a3e76618-4cb5-4d54-a71c-da4fb8ba571b@eisentraut.org

Author: petere

doc/src/sgml/installation.sgml

@@ -55,9 +55,11 @@
       </indexterm>
 
       Alternatively, <productname>PostgreSQL</productname> can be built using
-      <ulink url="https://mesonbuild.com/">Meson</ulink>.  This is currently
-      experimental.  If you choose to use
-      <application>Meson</application>, then you don't need
+      <ulink url="https://mesonbuild.com/">Meson</ulink>.  This is the only
+      option for building <productname>PostgreSQL</productname> on Windows
+      using <productname>Visual Studio</productname>.  For other platforms,
+      using <application>Meson</application> is currently experimental.  If
+      you choose to use <application>Meson</application>, then you don't need
       <acronym>GNU</acronym> <application>make</application>, but the other
       requirements below still apply.
      </para>