gh-139707: Specify winreg, msvcrt and winsound module availability in docs (GH-140429)

Doc/library/msvcrt.rst

@@ -22,6 +22,8 @@ api. The normal API deals only with ASCII characters and is of limited use
 for internationalized applications. The wide char API should be used where
 ever possible.
 
+.. availability:: Windows.
+
 .. versionchanged:: 3.3
    Operations in this module now raise :exc:`OSError` where :exc:`IOError`
    was raised.

Doc/library/winreg.rst

@@ -14,6 +14,8 @@ integer as the registry handle, a :ref:`handle object <handle-object>` is used
 to ensure that the handles are closed correctly, even if the programmer neglects
 to explicitly close them.
 
+.. availability:: Windows.
+
 .. _exception-changed:
 
 .. versionchanged:: 3.3

Doc/library/winsound.rst

@@ -13,6 +13,8 @@
 The :mod:`winsound` module provides access to the basic sound-playing machinery
 provided by Windows platforms.  It includes functions and several constants.
 
+.. availability:: Windows.
+
 
 .. function:: Beep(frequency, duration)