EXPORT_PACKAGE_DEPENDENCIES: Add experimental feature gate

Some design concerns have been raised after trying the 3.29 release
candidates.  Avoid committing to a stable public interface for now.

Issue: #25767

Help/command/export.rst

@@ -120,7 +120,9 @@ associated with the export ``<export-name>`` using the ``EXPORT`` option
 of the :command:`install(TARGETS)` command.
 
 ``EXPORT_PACKAGE_DEPENDENCIES``
-  .. versionadded:: 3.29
+  .. note::
+
+    Experimental. Gated by ``CMAKE_EXPERIMENTAL_EXPORT_PACKAGE_DEPENDENCIES``.
 
   Specify that :command:`find_dependency` calls should be exported. See
   :command:`install(EXPORT)` for details on how this works.
@@ -180,6 +182,10 @@ Configuring Exports
 Configure the parameters of an export. The arguments are as follows:
 
 ``PACKAGE_DEPENDENCY <dep>``
+  .. note::
+
+    Experimental. Gated by ``CMAKE_EXPERIMENTAL_EXPORT_PACKAGE_DEPENDENCIES``.
+
   Specify a package dependency to configure. This changes how
   :command:`find_dependency` calls are written during
   :command:`export(EXPORT)` and :command:`install(EXPORT)`. ``<dep>`` is the

Help/command/install.rst

@@ -853,7 +853,9 @@ Signatures
     targets in the export set will support being imported in consuming targets.
 
   ``EXPORT_PACKAGE_DEPENDENCIES``
-    .. versionadded:: 3.29
+    .. note::
+
+      Experimental. Gated by ``CMAKE_EXPERIMENTAL_EXPORT_PACKAGE_DEPENDENCIES``.
 
     Specify that :command:`find_dependency` calls should be exported. If this
     argument is specified, CMake examines all targets in the export set and