Help: Mention in find_package that cmake-gui step is Windows-only (#14781)

Recently used cmake-gui locations are searched only on Windows because
the Windows registry is used to record the values.  This behavior is
historical and may be removed by a policy in the future so rather than
implementing it on other platforms simply document the current behavior.

Help/command/find_package.rst

@@ -269,6 +269,7 @@ enabled.
    This can be skipped if ``NO_CMAKE_BUILDS_PATH`` is passed.  It is intended
    for the case when a user is building multiple dependent projects one
    after another.
+   (This step is implemented only on Windows.)
 
 6. Search paths stored in the CMake :ref:`User Package Registry`.
    This can be skipped if ``NO_CMAKE_PACKAGE_REGISTRY`` is passed.