Help: Fix CMAKE_HOST_WIN32 documentation

The documentation falsely claims `CMAKE_HOST_WIN32` is `true` on Cygwin.
Since commit v2.8.4~153^2 (Cygwin: Do not define 'WIN32', 2010-12-17) we
do not define `CMAKE_HOST_WIN32` on Cygwin either.

Help/variable/CMAKE_HOST_WIN32.rst

@@ -1,6 +1,6 @@
 CMAKE_HOST_WIN32
 ----------------
 
-``True`` on Windows systems, including Win64.
+``True`` if the host system is running Windows, including Windows 64-bit and MSYS.
 
-Set to ``true`` when the host system is Windows and on Cygwin.
+Set to ``false`` on Cygwin.