Help: Document experimental feature gate affecting CXX_SCAN_FOR_MODULES

Also drop the release note for `CXX_SCAN_FOR_MODULES`, as the rest of
the C++ modules support has not yet been mentioned in release notes
(since it is still experimental).

Help/prop_sf/CXX_SCAN_FOR_MODULES.rst

@@ -17,3 +17,8 @@ Note that scanning is only performed if C++20 or higher is enabled for the
 target and the source uses the ``CXX`` language.  Scanning for modules in
 sources belonging to file sets of type ``CXX_MODULES`` and
 ``CXX_MODULES_HEADER_UNITS`` is always performed.
+
+.. note ::
+
+  This setting is meaningful only when experimental support for C++ modules
+  has been enabled by the ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API`` gate.

Help/prop_tgt/CXX_SCAN_FOR_MODULES.rst

@@ -20,3 +20,8 @@ scan the target's ``CXX`` sources at build time.
 Note that scanning is only performed if C++20 or higher is enabled for the
 target.  Scanning for modules in the target's sources belonging to file sets
 of type ``CXX_MODULES`` and ``CXX_MODULES_HEADER_UNITS`` is always performed.
+
+.. note ::
+
+  This setting is meaningful only when experimental support for C++ modules
+  has been enabled by the ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API`` gate.

Help/release/dev/cxx-scanning-properties.rst

@@ -1,5 +0,0 @@
-cxx-scanning-properties
------------------------
-
-* The :prop_tgt:`CXX_SCAN_FOR_MODULES` target and source file properties may
-  be used to enable or disable scanning for C++ module dependencies.

Help/variable/CMAKE_CXX_SCAN_FOR_MODULES.rst

@@ -8,3 +8,8 @@ Whether to scan C++ source files for module dependencies.
 This variable is used to initialize the :prop_tgt:`CXX_SCAN_FOR_MODULES`
 property on all the targets.  See that target property for additional
 information.
+
+.. note ::
+
+  This setting is meaningful only when experimental support for C++ modules
+  has been enabled by the ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API`` gate.