cmExperimental: gate build database support behind a flag

Given that the feature currently only supports C++ sources and is not
formally accepted by ISO yet, gate it behind a flag.

Help/dev/experimental.rst

@@ -80,3 +80,24 @@ When activated, this experimental feature provides the following:
 
 .. _CPS: https://cps-org.github.io/cps/
 .. |CPS| replace:: Common Package Specification
+
+Build database support
+======================
+
+In order to activate support for exporting build databases, set
+
+* variable ``CMAKE_EXPERIMENTAL_EXPORT_BUILD_DATABASE`` to
+* value ``4bd552e2-b7fb-429a-ab23-c83ef53f3f13``.
+
+This UUID may change in future versions of CMake.  Be sure to use the value
+documented here by the source tree of the version of CMake with which you are
+experimenting.
+
+When activated, this experimental feature provides the following:
+
+* The :prop_tgt:`EXPORT_BUILD_DATABASE` target property and its initializing
+  variable :variable:`CMAKE_EXPORT_BUILD_DATABASE` and environment variable
+  :envvar:`CMAKE_EXPORT_BUILD_DATABASE`.
+
+* Targets with the property set to a true value will have their C++ build
+  information exported to the build database.

Help/envvar/CMAKE_EXPORT_BUILD_DATABASE.rst

@@ -9,3 +9,9 @@ The default value for :variable:`CMAKE_EXPORT_BUILD_DATABASE` when there is no
 explicit configuration given on the first run while creating a new build tree.
 On later runs in an existing build tree the value persists in the cache as
 :variable:`CMAKE_EXPORT_BUILD_DATABASE`.
+
+.. note ::
+
+   This variable is meaningful only when experimental support for build
+   databases has been enabled by the
+   ``CMAKE_EXPERIMENTAL_EXPORT_BUILD_DATABASE`` gate.

Help/prop_tgt/EXPORT_BUILD_DATABASE.rst

@@ -7,3 +7,9 @@ Enable/Disable output of a build database for a target.
 
 This property is initialized by the value of the variable
 :variable:`CMAKE_EXPORT_BUILD_DATABASE` if it is set when a target is created.
+
+.. note ::
+
+   This property is meaningful only when experimental support for build
+   databases has been enabled by the
+   ``CMAKE_EXPERIMENTAL_EXPORT_BUILD_DATABASE`` gate.

Help/variable/CMAKE_EXPORT_BUILD_DATABASE.rst

@@ -3,6 +3,12 @@ CMAKE_EXPORT_BUILD_DATABASE
 
 .. versionadded:: 3.31
 
+.. note ::
+
+   This variable is meaningful only when experimental support for build
+   databases has been enabled by the
+   ``CMAKE_EXPERIMENTAL_EXPORT_BUILD_DATABASE`` gate.
+
 Enable/Disable output of module compile commands during the build.
 
 If enabled, generates a ``build_database.json`` file containing the